Wes Streeting will plunge Labour into civil war today by quitting the Cabinet to mount a leadership challenge against Keir Starmer.

The Health Secretary is ready to fire the starting gun this morning following yesterday's face-to-face showdown with the Prime Minister which lasted just 16 minutes.

His plans triggered panic in Downing Street and fury on the Labour Left, where MPs are even considering Ed Miliband as they scramble to unite behind a candidate of their own.

Sir Keir was reduced to pleading with his MPs to step back from the brink of a leadership contest, which he warned would cause 'chaos' and paralyse' the Government for months. 

During a series of private meetings in the Commons, he told them: 'We can't let a leadership contest plunge us into chaos and a challenge would 100 per cent do that.'

Allies of the PM insist he will stand and fight for his job in what is likely to become a three or even four-way contest for the keys to No 10.

Greater Manchester mayor Andy Burnham is expected to signal his own ambition to succeed Sir Keir today. But it remains unclear whether he can find a route back to Parliament in time.

One union leader accused Mr Streeting of attempting a 'coup' by launching a leadership bid before Mr Burnham is able to stand.

MPs on the Left of the party were last night debating whether to back Angela Rayner or Mr Miliband if Mr Burnham is locked out of the race.

Mr Miliband was emerging as favourite despite being rejected by the public at the 2015 election, although his wife Justine Thornton is thought to be against the idea of him running for the leadership again.

Other potential candidates include deputy leader Lucy Powell and Culture Secretary Lisa Nandy. 

Cabinet Office minister Darren Jones is being encouraged to run as a 'continuity Starmer' candidate if the PM falls by the wayside. Former Special Forces officer Al Carns is also considering entering the race.

The extraordinary events unfolded on the day of the State Opening of Parliament, with the King reading out the legislative agenda of a Prime Minister whose downfall was being plotted by his own MPs. Kemi Badenoch said Sir Keir was 'in office, but not in power'.

In a blistering performance in the Commons, the Conservative leader warned that Britain will be 'subjected to months of peacocking by leadership candidates while the country is not being governed'.

She added: 'In the past 48 hours, nearly 100 Labour MPs have called for the Prime Minister to resign. Four ministers have quit. 

'It is clear that his authority has gone and that he will not be able to deliver what little there is in this King's Speech. This is a Government less than two years in office who have already run out of ideas and run out of road.'

Pressure has been mounting on Sir Keir since last week's disastrous local elections, which saw Labour lose 1,500 council seats in England and trail in third place in Scotland and Wales.

Downing Street had thought it had seen off the threat from Mr Streeting after he failed to mount an immediate challenge. Government sources were even goading him yesterday morning, suggesting he had 'bottled it'.

The PM faced another body blow when Labour-supporting trade unions told him to set out a timetable for his departure, saying it was 'clear that the Prime Minister will not lead Labour into the next election'. 

In a joint statement, the 11 unions said 'Labour cannot continue on its current path' and needs 'a fundamental change of direction' to 're-orient Labour back to working people'.

Minutes later, Mr Streeting strode into No 10 for what Downing Street described as a 'coffee' to 'discuss his concerns' about last week's election results and the direction of the party.

He emerged stony-faced 16 minutes later and stalked off without speaking to waiting reporters. Labour's warring factions had agreed a tentative 24-hour truce to avoid the potential embarrassment to the monarch by launching a leadership challenge on the day of the King's Speech.

But as Charles was making his way to Westminster, allies of Mr Streeting confirmed he is planning to resign today.

Downing Street insisted the PM had 'full confidence' in the Health Secretary. But privately, there was fury at his decision to challenge for the top job. 

One ally of Sir Keir said: 'This will open up a Pandora's Box from which Labour will never recover. If it ends up with Ed in No 10 then it is all over.'

Mr Streeting is on the Right of the party and is viewed with deep suspicion by many on the Left.

In the Commons, Cabinet ministers snubbed Mr Streeting, with Education Secretary Bridget Phillipson ignoring his attempt to strike up conversation.

Sir Keir's remaining Cabinet supporters were working with the whips last night to try to pick off Mr Streeting's supporters. To launch a formal challenge, he will need the support of at least 81 Labour MPs. Some MPs close to Sir Keir believe he could struggle to reach the threshold. 

Under Labour's rules, other challengers could then enter the contest if they can also reach the threshold of support. Sir Keir's name would be entered automatically.

Mr Streeting's allies are believed to have contacted MPs asking for their support, it was reported last night. They are said to have suggested the politicians could transfer their vote to another candidate after a contest is triggered. 

Supporters of Mr Burnham are lobbying Labour's ruling National Executive Committee to agree an extended timetable for the leadership contest to give him time to fight a by-election.

But any delay to accommodate Mr Burnham could leave Britain with a lame-duck Prime Minister for months. And it remains unclear whether Mr Burnham would win a by-election. 

Labour peer Lord Hutton warned that any new leader proposing a radical change of direction would have to call an election to get their own mandate.

The former Cabinet minister told LBC: 'With the bond markets in the state that they are in, I think there is a huge risk that things get really worse if we choose the wrong leader.'