Mandelson was given developed vetting status despite failing checks carried out by the agency responsible for assessing security clearances.

Sir Keir Starmer will battle to save his job in Parliament on Monday by setting out further details of the “unforgivable” error by officials in not telling him Peter Mandelson had failed his security vetting.

In a Commons statement, Sir Keir will be faced with allegations he misled Parliament after telling MPs the proper process had been followed in appointing Lord Mandelson to the post of ambassador to the US, insisting he had been kept in the dark about the peer being red-flagged by security experts.

Sir Keir effectively fired the Foreign Office’s top official, Sir Olly Robbins, last week after it emerged Lord Mandelson had been given developed vetting (DV) status despite failing checks carried out by the agency responsible for assessing security clearances.

Whitehall veteran Sir Olly is expected to give his own account to MPs on Tuesday at the Foreign Affairs Committee.

The scandal has fuelled calls for Sir Keir to resign, both from opposition parties and from his critics within the Labour movement who already fear an electoral bloodbath for the party in May’s contests in English councils and the Scottish and Welsh parliaments.

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The Prime Minister’s defence will be to blame officials for not telling him or the then foreign secretary David Lammy that UK Security Vetting (UKSV) had not cleared Lord Mandelson.

A statement issued by No 10 on Sunday night said that although civil servants, rather than ministers, make decisions on vetting and clearance, there was nothing in the law to prevent ministers from being told.