Discovery for developers: about the application programming interface (API)

If you would like access to our API, please contact us and include the IP address from which you will be sending requests to our API.

Discovery holds more than 32 million descriptions of records held by The National Archives and more than 2,500 archives and institutions across the United Kingdom as well as a much smaller number of archives around the world.

The information in Discovery is made up of record descriptions provided by or derived from the catalogues of the different archives. Although some of The National Archives’ records have been digitised and can be read online, Discovery can’t search the words within them – only their description and title.

Our API allows developers to query the search engine and database within the Discovery service application programmatically, and returns results in XML or Json for further processing. The service is offered as a beta with some functionality still to be developed. In the meantime we welcome feedback on the functionality in this release.

Terms of use

Statement of intent

Our Discovery application programming interface (API) is designed to maximise access to the information held in our Discovery service catalogue.

You are welcome to use the information and the images for personal use, educational purposes or commercial use under the Open Government Licence.

Best practice

Please tell us about your use of the API by emailing us a link to your websites and describing how you are using the API service. Also, please provide feedback about your experience of using the API so that we can work to improve the service.

Do not make an unreasonable number of API calls or use the API in a way which significantly compromises the experience of its other users. As a guideline, you should make no more than 3000 API calls per day at a rate of no more frequently than one request per second. We may choose to limit the number of API calls more formally in the future.

Report any concern you have over copyright to us.

Terms and conditions

If you make a request to this service you are deemed to have accepted the terms and conditions listed here.

You may not use The National Archives’ logo on your website without our specific written permission.

Our Discovery service is still under development. Please do not cache or store any content returned by the API.

You will not use The National Archives’ content or API service for any illegal or defamatory purpose of any nature. You will not use the API service to juxtapose our content with any illegal or defamatory material of any nature.

You understand and agree that The National Archives will not provide any technical support services in connection with any use of the API. We do not guarantee availability of the API service.

The National Archives reserves the right to extend or alter these terms and conditions at any time.

Understanding the Discovery service catalogue

Note that the following information relates to the content, metadata and structure of The National Archives’ catalogue dataset; other catalogues and datasets within Discovery may be different.

A little understanding of the structure of the Discovery service catalogue will help with the API methods below. Our catalogue dataset is organised hierarchically to reflect the origin and structure of the records. There are seven levels in the catalogue, ranging from ‘department’ at the top of the tree to pieces and, occasionally items at the bottom:

  1. department – a government department, agency or body that creates the records
  2. division – administrative section of a department, when appropriate
  3. series – the main grouping of records with a common function or subject
  4. sub-series – smaller groupings of records with a common function or subject
  5. sub sub-series – smaller groupings of records with a common function or subject
  6. piece – it is not a single piece of paper. A piece can be a box, volume or file
  7. item – it is a part of a piece. It can be a bundle, a single document, or a letter and so on

Every level of description in the hierarchy is described within a catalogue entry according to the international standard ISAD (G). The dataset follows its rules for multi-level catalogues (specificity, relevancy, hierarchy position and non-repetition of information at different levels).

When using the API methods below, you’ll find that some of the normally ‘invisible’ levels (levels which are present but not reflected in the citable reference, such as ‘division’) will become relevant and may be initially confusing when using methods like Children which traverse the levels of the catalogue.

Every record in Discovery is an Information Asset and is uniquely referenced by its Information Asset Identity/Identifier (IAID). Some of the Information Assets have children, and this parent-child relationship defines the hierarchy.

The API uses two methods:


Search method is processed by our Discovery service search engine. It offers a basic search where you can enter a single or multiple search terms. The default is to search for any of the words. To search for a phrase use double quotation marks.

Note that the Discovery service database comprises three primary record collections:

    • The National Archives’ catalogue
    • catalogues from other archives
    • records compiled from record creator collections

Search will return results and individual information assets from all three collections.

Information assets

The remainder of the methods take an Information Asset IAID and return results directly from our Discovery service database.

The information assets method returns metadata for

    • The National Archives’ catalogue
    • catalogues from other archives
    • records compiled from record creator collections
    • archive contacts

You can return selected metadata for all four source types but at the moment most of information asset methods only return results from The National Archives’ catalogue.

Operations at

This page describes the service operations at this endpoint.

The information can be accessed using an HTTP GET request’

Uniform resource identifier (URI) Description
search/page/{pageid}/{query} service at

Standard search


Exact match using quotes


Search for reference


Search for terms within dates


Search for terms within a specific archive’s catalogue

Page/{PageID}/query={QUERY};repositorycodes={archon code}

informationassets/{id} service at{ID}
informationassets/{ID}/children/Page/{pageid} service at{ID}/children/page/{PageID}[The National Archives collection only]
informationassets/{ID}/totalchildren service at{ID}/totalchildren

[The National Archives collection only]

informationassets /{ID}/parent service at{id}/parent[The National Archives collection only]

Note that a page comprises 30 results.

Some examples: the results are returned as XML by default

page 1 of search results for Nelson held by The National Archives;repositorycodes=66

You can get the archon code from the archive contact page. Search for this from

page 1 of a list of records containing the National Archives reference CO 28 [note that to improve the results you can enter the reference in quotes and maybe add repository code]
page 1 of a list of results for Cranham held by Gloucestershire Archives between 1900 and 1920;repositorycodes=40;datefrom=01-01-1900;dateto=31-12-1920
search for Gloucestershire Archives reference P103a acc 11059, acc 11117,%20acc%2011117
information Asset details for C1007252, an example from The National Archives
information Asset details for f313499d-0a90-41c6-9c2b-1b4633b68c27, an example from another archive catalogue
information Asset details for N13861434, an example of a record creator record
information Asset details for A13532926, contact details for Gloucestershire Archives
Parent Information Asset for IAID=C57915 [The National Archives collection only]
page 1 of search results for Titanic

Querying The National Archives catalogue directly

PCOM 3 Example

Some series in The National Archives’ catalogue contain structured data, for example PCOM 3 (Licences for early release from prison) has a standardised ScopeContent description across the entire series which could be extracted and parsed into a dataset.

Here’s how you might extract this data (please read the table across from left-right for the step-by-step process):

first we need to identify the IAID of the container series, PCOM 3. The Search method allows searching for document references, so the following query will return records matching “PCOM 3”:
checking the CitableReference values we see that one of the results is for an information asset which represents the series PCOM 3. We see that the IAID for this series is C11498. we can find out how many child information assets PCOM 3 has by supplying its IAID to the total children method:

which shows that 770 information assets have PCOM 3 as their parent. to find out more about those children we use the children method:

this returns the first page (30 records) of child information assets for PCOM 3. The IAID of the first child IA is C2389021, and its citable reference is “PCOM 3/1”. The SourceLevelId value shows that this Information Asset is at level 6 of the catalogue (i.e. a piece). The descriptions may be more granular still if the piece has been itemised further. Let’s check if PCOM 3/1 has further details by checking if it has any children
this reveals that PCOM 3/1 has been itemised and has 100 child Information Assets. we need to get the IAIDs of those children and then iterate through them, sending them to the InformationAssets method to return the detailed descriptions. There are 30 IAIDs per page

This returns InformationAssetIdentity objects which are a brief summary of the Information Asset, but include the IAID which can be sent to the InformationAssets method to return the detailed Information Asset information. The first child of PCOM 3/1 is PCOM 3/1/1 with IAID C10127419

C 3 example

C 3 is another series where the information has been entered in a standardised format which could be parsed into a structured dataset.

get the IAID for C 3:
the IAID for C 3 is C3566 get a list of its Children:

the children of C 3 are at level 6, piece level (though confusingly, the citable reference of these pieces is in the format c3/x/x, which would more commonly be an item reference). for each child we need to:

  • confirm if it has children
  • fetch the detailed Information Asset object
  • if there are Children, call this procedure recursively for them

the method calls we need to make for each child IA are therefore:

if TotalChildren > 0, recurse