Sir Richard Dearlove has declared his astonishment that David Lammy was apparently kept in the dark about Peter Mandelson's unsuccessful security vetting, branding the scandal a "complete mess".
Speaking to GB News, the former head of MI6 said: "I cannot believe that a Permanent-Under Secretary, when he got the results of the DV, didn't ring up his minister, who he talks to every day, and say to him, 'Look, Mr Lammy, minister, we have a problem, and we have to work out now how we're going to manage that problem'."
Sir Richard was scathing about the Prime Minister's original decision to select Lord Mandelson for the Washington posting.
He said: "The problem ultimately is caused by the Prime Minister choosing an ambassador with a known integrity problem.
"Everybody understood that about Peter Mandelson, if you look at his past record."
The former spy chief acknowledged Lord Mandelson's capabilities but pointed to his troubled history, describing him as having "a very dodgy series of relationships" during his time as a prominent New Labour figure.
Sir Richard said: "It was a bad choice in the first place. It was an appalling choice in the first place."
He revealed he had warned on the day the appointment was announced that it would likely create "massive problems" for the Government.
Richard Dearlove has criticised Sir Keir Starmer in light of the latest Peter Mandelson row
|PA / GB NEWS
Sir Richard raised pointed questions about whether Olly Robbins, the Foreign Office's Permanent Under-Secretary, had adhered to established protocols when handling the vetting outcome.
He said: "Did Olly Robbins really take it on himself to not tell anybody and decide, as the Permanent Under-Secretary of the Foreign Office, that the risk was manageable?"
The former intelligence chief characterised the entire situation as deeply problematic, regardless of how one interprets events.
He said: "I mean, whichever way you look at it, it's a mess."
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Two of Sir Keir Starmer's aides in the Civil Service knew Lord Mandelson had failed vetting weeks before the Prime Minister | GETTYSir Richard concluded that the affair represents either a serious lapse in judgement or a fundamental breakdown in proper procedures.
He stressed that although the specifics of any vetting process must remain strictly confidential, the outcome itself carries considerable weight.
The ex-MI6 boss said: "There is confusion about the results of a vetting process. You do not divulge the detail of the vetting process. You can't explain the detail.
"It's all high confidentiality, but the results of the vetting process without explanation are significant."
Sir Richard told GB News that that he 'cannot believe' that the Foreign Secretary was not informed that Peter Mandelson had failed security vetting
|GB NEWS
The former MI6 head expressed incredulity that someone of Robbins's experience and competence would not have immediately consulted Mr Lammy upon learning of the failed vetting, suggesting the Foreign Secretary could then have briefed the Prime Minister on managing the risk.
Defending Sir Keir, Labour MP Douglas Alexander told GB News that it is "simply not true" that the PM "knowingly misled parliament and lied to the public".
He said: "The fact is, the Prime Minister is being alleged by the Leader of the Opposition to have knowingly misled parliament and lied to the public.
"That is not true, and the Prime Minister will set out why that's not true when he speaks at the dispatch box of the House of Commons just behind me this afternoon."