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The story of how Peter Mandelson failed his UK security vetting before he took up his post as ambassador to the US – and the overturning of the decision that he should not be given clearance – is full of the acronyms of the British national security apparatus and the archaic language used to describe parliamentary process.

Here are the key terms to understand about the story, as the prime minister, Keir Starmer, faces a vital appearance in the House of Commons in London about who in the government knew what, when.

Developed Vetting (DV)

At the heart of the story is Mandelson’s application for a level of security clearance known as “developed vetting” (DV) made after his appointment as ambassador had been announced.

According to a government guide to security clearance levels, officials in roles that require them to have “frequent and uncontrolled access” to top secret material and assets need to have DV. That could mean sensitive areas of government buildings as well as classified information.

In the Foreign Office, it is routine for civil servants – including those in much more junior roles than Britain’s most high profile diplomatic posting – to have DV clearance. To get DV, officials are subjected to personal and often quite intrusive interviews; they have to fill out questionnaires and provide referees who are asked about the character and history of the applicant.

Security officials complete a form with three options: green, “clearance approved or remains valid”; yellow, “clearance approved or remains valid with risk management”, which might include later follow-up checks or restriction from working on certain policy areas; or red, “clearance denied or withdrawn”. In Mandelson’s case, they chose the third option to deny clearance.

Politicians, however, are not vetted: it is deemed their democratic position gives them the right to access the sensitive materials necessary to fulfil their duties.

United Kingdom Security Vetting (UKSV)

For most civil servants, vetting processes are carried out by an agency, United Kingdom Security Vetting (UKSV). Those working in the intelligence community are vetted by the security and intelligence services, but the security services’ records are consulted as part of the UKSV process.

UKSV is a part of the Cabinet Office, with offices in York. Some of its staff are former police officers, who use their experience to judge whether or not applicants are lying during their interviews.

Given the probing nature of the questions – which, according to publicly available government documents, include queries on personal finances, business connections and sexual history – applicants are told that it is lying or hiding something they have done which is more likely to lead to them failing the vetting process, rather than a judgment on their conduct.

The reports produced by UKSV are highly secretive and never publicly disclosed, even to the applicant. Similarly, according to a ruling by the Information Commissioner, the “vetting decision framework”, a guide used by vetting officers about the factors to consider, is protected by the Cabinet Office from wider public scrutiny.

The Cabinet Office and the humble address

The release of a new batch of documents from the Epstein Files in January 2026 led to a fresh wave of stories about Mandelson and the extent of his friendship with the convicted child sex offender Jefrey Epstein. That prompted a debate in parliament, where the Conservatives introduced a binding motion, agreed by MPs, forcing the government to publish “all papers” related to Mandelson’s appointment as ambassador. This rarely used motion is called a humble address.

Officials in the Cabinet Office, the department at the centre of the British government which supports the prime minister and his chosen cabinet of ministers, were tasked with responding on behalf of the government to comply with the humble address.

It was during this process that documents were sought from UKSV, which led to senior officials in the Cabinet Office learning in March that Mandelson had failed his vetting. But, as the Guardian revealed on Friday, several weeks passed before they told Keir Starmer. Government sources say this was owing to them seeking legal advice on whether they could inform the prime minister.

Intelligence and security committee (ISC)

Given the sensitivity of some of the papers within the scope of the humble address, including those relating to Mandelson’s vetting, MPs agreed any documents that could be prejudicial to the UK’s national security or international relations should first be given by the Cabinet Office to a trusted parliamentary committee.

The nine-person cross-party group, called the intelligence and security committee (ISC), routinely oversees the work of the British intelligence services. Its members consist of MPs and peers, and include a former top admiral. They meet in private and are given access to top secret material.

Once parliament passed the humble address motion, officials in the Cabinet Office held talks with the ISC on who would be the ultimate decision maker on whether material referred to the committee could be made public. Typically, the ISC’s reports are only published with the authorisation of the prime minister, but in this instance, it was agreed the ISC would, alone, decide what the public could see.

After weeks of internal disagreement by officials on whether the UKSV material could be disclosed in what would be an “unprecedented” move, the Cabinet Office has said the documents about Mandelson’s vetting would be given to the ISC, but is yet to provide a timeframe.