More than half a dozen civil servants knew about the Peter Mandelson vetting controversy before Sir Keir Starmer was finally told about it.
The Government has admitted that seven people, plus an unknown number of security officials, found out before the Prime Minister that red flags had been raised during background checks on the US Ambassador.
The secret vetting file was first passed to Cat Little, Permanent Secretary to the Cabinet Office, as she is in charge of gathering documents from across Whitehall on the Mandelson scandal after MPs voted for them to be published under the so-called Humble Address process.
After she learned about it in late March, she told the country's top civil servant Dame Antonia Romeo and asked Government lawyers what they should do with the bombshell information.
Ms Little was questioned by MPs investigating the saga about just how widely shared it had been within Whitehall before she told Sir Keir about it a fortnight ago.
In a new letter to the Foreign Affairs Committee, the Cabinet Office replied: 'Between 25 March and 14 April, seven individuals (in addition to 'must know' security officials) were made aware of, through the compliance with the Humble Address process in the CO [Cabinet Office], the outcome of the vetting summary document.'
Just two days after Ms Little and Dame Antonia told the PM about the UK Security Vetting file on Lord Mandelson, details of it were leaked to the Guardian.
The Government has since said it is trying to find out who was responsible and the Foreign Office mandarin sacked for failing to tell Sir Keir about the concerns raised, Sir Olly Robbins, has demanded prosecutions for the 'grievous breach of national security'.
The secret vetting file was first passed to Cat Little, pictured, as she is in charge of gathering documents from across Whitehall on the Mandelson scandal
Asked by MPs what the punishment would be for leaking such sensitive information, the Cabinet Office said: 'Our legal advisers advise that the maximum penalty on conviction is two years' imprisonment and/or an unlimited fine.'
Shadow Cabinet Office minister Mike Wood said: 'Either Keir Starmer lied about Mandelson's vetting, or he is totally incompetent. Or both.
'If we really believe the PM was the last to know the truth, what does that tell us about how he is viewed by leading civil servants?
'Starmer has misled Parliament and the public, and tried to blame everyone but himself for this appalling scandal. His sordid decision to force his own MPs to block an inquiry will not save him from his inevitable fate at the hands of voters.'
A Government spokesman said: 'We do not comment on live leak investigations.'