Kemi Badenoch is urging Labour MPs to put country before party and back a sleaze probe into Keir Starmer's Mandelson 'lies'.

MPs will vote on Tuesday evening on opposition calls for an inquiry into whether the Prime Minister has lied to Parliament about his disastrous decision to appoint Peter Mandelson as US ambassador.

Defeat for Sir Keir would throw his floundering government into a tailspin and could eventually see him forced from office.

Downing Street launched a major arm-twisting operation on Monday night to save him, including warning Labour MPs they are likely to face a three-line whip to fall in line.

But writing in the Daily Mail on Tuesday, Mrs Badenoch says MPs of all stripes have a duty to hold Sir Keir to account for treating Parliament with 'contempt'. 

She tells Labour backbenchers they are 'not in the Commons simply to protect the PM from embarrassment'.

'Labour MPs now face a test of their own,' the Tory leader said.

'They can circle the wagons, obey the Whips and tell themselves this is just politics. Or they can remember they are MPs before they are members of the Labour Party.'

Sir Keir faces a day of torment over his handling of the Mandelson scandal. 

He is accused of misleading Parliament by repeatedly claiming that 'full due process' was followed at all times, despite the fact that Mandelson was given the job before he was security vetted.

Mrs Badenoch also questioned Sir Keir's claim to MPs that 'no pressure whatsoever' was applied to officials to push through Mandelson's appointment.

Former Foreign Office chief Sir Philip Barton is expected to confirm he warned against sending Mandelson to Washington when he appears before MPs on Tuesday morning. 

He is also expected to back his successor, Sir Olly Robbins, who said officials were 'under constant pressure' from No 10 to sign off the appointment.

In a bombshell intervention last night, former Foreign Office security chief Ian Collard agreed with Sir Olly's claim that there had been pressure from Downing Street to carry out Mandelson's vetting quickly.

No 10 is also braced for the PM's former chief of staff, Morgan McSweeney, to give evidence on the scandal this morning.

Opposition MPs are today seeking an investigation by the Commons Privileges Committee into whether Sir Keir misled Parliament. 

In a rare move, Commons Speaker Sir Lindsay Hoyle granted permission for an emergency debate on the issue, saying it had been backed by 'numerous MPs from across the House'.

It is the same process used by Sir Keir to hound Boris Johnson out of Parliament over Partygate, but the PM insisted his own case was 'completely different'. 

He told Sky News the Government had 'huge amounts of transparency going on' – and branded today's Commons vote a 'political stunt'.

Addressing the Parliamentary Labour Party last night, he pleaded with his mutinous MPs to back him. 'Tomorrow is pure politics and we need to stand together against it,' he said. 'It's important to see the bigger picture here. They want to stop this Labour Government. When we stick together and fight together we are so much stronger.'

Sir Keir has struggled to escape the backlash for pressing ahead with Mandelson's appointment despite being warned in writing that he had a 'particularly close' relationship with paedophile Jeffrey Epstein and questionable business links in Russia and China.

The PM has tried to blame officials, sacking Sir Olly this month after discovering that the appointment was pushed through against the advice of UK Security Vetting without his knowledge. However, he has struggled to convince MPs and even members of his Cabinet of his story.

Angela Rayner appeared to throw the PM a lifeline last night, with allies suggesting she will not vote for the inquiry.

Lib Dem MP Lisa Smart said last night: 'Labour MPs must put principle before party and vote to refer Keir Starmer to the Privileges Committee.'

Deputy Prime Minister David Lammy yesterday became the latest senior figure to confirm that he warned the PM against appointing Mandelson.

'There were worries at the time,' he told LBC. 'And there was a discussion within government.'

MPs are usually given a free vote on matters of whether Parliament has been told the truth.

Tory grandee Sir Desmond Swayne said it would be 'an abomination' if Labour MPs were ordered to fall into line to save the PM's skin over a question of whether he told the truth.

But panicking Labour Whips are expected to warn MPs they risk suspension from the party if they fail to back Sir Keir today.

And No 10 took the unusual step of selectively releasing a letter from the Mandelson files which they believe strengthens the PM's case that he told the truth.

The letter from former permanent secretary Sir Chris Wormald said an investigation into Mandelson's appointment had found that 'appropriate processes were followed'. But Mrs Badenoch said it was 'irrelevant' as it was already clear that Sir Chris's predecessor Simon Case had advised the PM not to announce the appointment until he had been vetted for security – advice that was ignored.

MPs were told that tens of thousands of files relating to Mandelson – which Parliament demanded in February – will not be released until after the local elections.