Reform UK has threatened to “carpet bomb” Labour MPs’ constituencies with campaign material if they fail to back a sleaze probe into Sir Keir Starmer. The Prime Minister faces a grave day of peril on Tuesday when MPs decide if to open a Privileges Commitee investigation into whether the Labour leader misled Parliament over the Lord Mandelson scandal.

Sir Keir insisted in the Commons that “full due process” was followed over his catastrophic decision to hand the former Labour grandee the plum job as British ambassador to Washington. The PM also said that “no pressure whatsoever” was put on Foreign Office civil servants over Lord Mandelson’s security vetting, contrary to evidence from former departmental chief Sir Olly Robbins. Commons Speaker Sir Lindsay Hoyle is allowing a debate on the issue to take place following pressure from Sir Keir’s leading critics, including Tory leader Kemi Badenoch.

MPs will then vote on the matter, with Labour set to whip its members to vote against despite pleas from opposition parties to make it a free vote.

But they are set to face pressure from both sides after Reform home affairs spokesman Zia Yusuf wrote on social media site X on Monday: “Friendly note to Labour MPs ahead of the vote tomorrow to decide if Starmer should face an ethics probe: If you vote against it, Reform will carpet bomb your constituency to ensure all your constituents know you voted to save the most unpopular PM of all time. Vote wisely.”

Sir Keir then dramatically fired Sir Olly, former permanent under secretary at the Foreign Office, when concerns over Lord Mandelson’s security clearance came to light.

The mandarin said he was simply following procedure around vetting and that there was a “constant pressure” from Downing Street to approve the ex-ambassador’s clearance.

Before the vote, Mr McSweeney will give bombshell evidence to the Foreign Affairs Committee, which could mean yet more disastrous revelations for Sir Keir.