Conservative MPs rounded on Boris Johnson after it emerged that the US ambassador had decided to resign in the wake of the senior Tory's lack of support.
Sir Kim Darroch said his job had become “impossible” after the leak of diplomatic memos highly critical of Donald Trump’s administration.
His decision is understood to have been made after Mr Johnson repeatedly refused to support him during a televised Tory leadership debate on Wednesday.
Meanwhile, Labour denied claims senior figures in the party interfered with anti-semitism complaints as a BBC Panorama documentary was set to air at 9pm.
A Labour spokesman accused the broadcaster of “pre-determining” the outcome of its investigation – while the broadcaster claimed the party was “criticising a programme they have not seen”.
See below for live updates
A juicy story to start the day.
John Major says he will launch a judicial review to stop Boris Johnson shutting down parliament to force through a “totally unacceptable” no-deal Brexit.
The former Conservative prime minister condemned the Tory leadership favourite for failing to rule out prorogation – which he likened to the actions of Charles 1 in the 1640s.
Tory Brexiteers were guilty of championing parliamentary sovereignty “except when it is inconvenient for Mr Johnson”, Sir John said.
Labour should change its rules to automatically throw out members if there is clear evidence of antisemitism, a senior shadow minister has urged.
Sir Keir Starmer said the party had to do "whatever is necessary" to win back critics who had deserted Labour over its handling of anti-Semitism claims.
His comments came after three senior peers quit the Labour whip in protest at the handling of the issue under Jeremy Corbyn's leadership.
Labour is braced for further revelations in a potentially damaging BBC Panorama investigation on antisemitism in the party which is due to be aired on Wednesday night.
Lord Triesman, a former Labour general secretary and foreign office minister, said the party and its leadership are "institutionally anti-Semitic", while Lord Darzi and Lord Turnberg also quit the Labour whip.
Sir Keir said it was "very easy for people in leadership positions" in the shadow cabinet to "duck responsibility".
He told BBC Radio 4's Today programme: "I think we have got to take decisive action.
"Firstly, I would support a rule change that says you expel in clear cases of antisemitism automatically, just as we do for people who support another political party an election.
"Secondly, be very clear if you deny we've got a problem, that's part of the problem.
"Thirdly - and this is a cultural and attitude issue - now the Equality and Human Rights Commission is looking at the Labour party, we should throw open the books and say 'you've got access to anything, you have got access to any member of staff, we have made some process changes which have improved things but we have still got a problem, help us through this'.
"Many organisations circle the wagons when they are challenged. That is the wrong approach, we have got to be very, very open."
Pro-EU political parties are set to take a major step towards forming a national “Remain alliance” by working together with a new organisation designed to increase the number of anti-Brexit MPs.
Independent MP Heidi Allen, who quit the Conservative Party earlier this year, will launch the “Unite to Remain” group on Wednesday, Ben Kentish reveals.
Team Boris has hit back at John Major, with sources saying he has "gone completely bonkers" over threat of judicial review.
"He's clearly been driven completely mad by Brexit."
Ex-Labour MP John Woodcock has joined The Independents - *another* twist in the journey of The Independent Group/Change UK.
He joins Heidi Allen, Luciana Berger, Gavin Shuker and Angela Smith for the group, which is a "workers collective" rather than a political party.
It comes after Labour and Tory MPs quit their parties in February in protest over issues such as Brexit and handling of antisemitism by Jeremy Corbyn.
At first they sat as independent MPs, known as the Independent Group (TIG) before becoming Change UK to stand in the European Parliament elections.
Meanwhile, half of the splitters decided to leave Change UK - Chuka Umunna, Sarah Wollaston, Allen, Berger, Shuker and Smith.
Anna Soubry became the new Change UK leader, with Joan Ryan, Chris Leslie, Mike Gapes and Ann Coffey staying put.
However a legal challenge from the petitions website Change.org forced the group to change its name to The Independent Group for Change.
Umunna joined the Liberal Democrats while Wollaston remains an independent.
Britain's Next Prime Minister: The ITV Debate was watched by an average of 4.3m viewers. According to viewing figures from ITV, the debate had a peak audience of 4.7m viewers.
Boris Johnson has refused to say he would keep Sir Kim Darroch as Britain’s ambassador to the US if he became prime minister.
During a live television debate between the two Tory candidates to become the next prime minister, the former foreign secretary initially evaded the question and refused to commit himself on Sir Kim’s fate, but when pressed said he wouldn’t be “presumptuous”.
In a letter to Sir Simon McDonald, the permanent under secretary at the Foreign and Commonwealth Office, Sir Kim wrote: "Since the leak of official documents from this Embassy there has been a great deal of speculation surrounding my position and the duration of my remaining term as ambassador. I want to put an end to that speculation.
"The current situation is making it impossible for me to carry out my role as I would like.
"Although my posting is not due to end until the end of this year, I believe in the current circumstances the responsible course is to allow the appointment of a new ambassador.
"I am grateful to all those in the UK and the US, who have offered their support during this difficult few days. This has brought home to me the depth of friendship and close ties between our two countries. I have been deeply touched.
"I am also grateful to all those with whom I have worked over the last four decades, particularly my team here in the US. The professionalism and integrity of the British civil service is the envy of the world. I will leave it full of confidence that its values remain in safe hands."
Theresa May pays tribute to Sir Kim Darroch for his lifetime of service to the UK and says she offered him her full support. It is a matter of 'great regret' to her that he had to resign.
Ms May said: “The whole Cabinet rightly gave its full support to Sir Kim on Tuesday. Sir Kim has given a lifetime of service to the United Kingdom and we owe him an enormous debt of gratitude.
“Good government depends on public servants being able to give full and frank advice. I want all our public servants to have the confidence to be able to do that and I hope the House will reflect on the importance of defending our values and principles particularly when they are under pressure.”
Jeremy Corbyn says he regrets Sir Kim's departure and the criticism about him were "unfair and wrong".
He told PMQs: “I too regret the resignation of Sir Kim Darroch. I think the comments made about him were beyond unfair and wrong. I think he has given honourable and good service and he should be thanked for it.
“I think the whole House should join together in deeply regretting the feeling he has obviously got that he must resign at this moment.”
The Labour leader asks May about cuts to legal aid and whether it has helped her fight against burning injustices.
May says it is not only about access to the legal system and if Labour cared so much, they should have done more when they were in government.
Corbyn quotes a 71-year-old member of the public called Marcus, who is threatened with eviction and struggling to collect evidence to challenge the decision.
May says quarter of the Ministry of Justice budget is spent on legal aid - but that is only "one part of the picture".
