You can catch up on today's events in Parliament with Today in Parliament on BBC Radio 4 at 23:30 BST.
In the Commons, Theresa May said UK ambassador Sir Kim Darroch's departure was "a matter of deep regret" after the ambassador said it was "impossible" for him to continue.
Mrs May said Sir Kim had had the full backing of the cabinet and he was owed an "enormous debt of gratitude" for his "lifetime of service" to the UK.
Later, the leak was described as "malicious" by head of the diplomatic service Sir Simon McDonald.
He told the Commons' Foreign Affairs Committee it was the first time in his career that a head of state had refused to work with a British ambassador.
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BBC North America reporter
I suspect President Trump will certainly comment on Sir Kim Darroch's resignation at some point - usually he likes to celebrate the exercise of his power at moments like this.
And it's all very familiar. Donald Trump doesn't fire people, even in his own administration. What he does is make it so uncomfortable for them that they eventually resign of their own volition. We've seen it time and time again, and that's what happened here.
The whole row is getting some coverage in the US, but the reality is that this is viewed as just the latest in a long line of Trump social media spats. And the nation at large, now that this has basically resolved itself as far as they're concerned, will move on.
Theresa May's de facto deputy, Cabinet Office minister David Lidington, has told a Commons committee he is "enraged" by the Sir Kim Darroch row.
He said morale among the senior ranks of the civil service had taken a "very heavy blow" and the quality of government suffered if officials felt inhibited from giving frank advice to ministers.
Mr Lidington said: "Ultimately it's my decision whether I drive the car off a cliff, but if my civil servants see me heading towards Beachy Head, I do actually want them to come and tell me.
"Their job is to give me the frankest, the most honest advice possible, whether it's something that I would welcome hearing or not."
BBC News Channel
Foreign affairs committee chairman Tom Tugendhat earlier questioned the head of the diplomatic service on the whole row.
Now he tells the BBC: "The real problem from this is other embassies around the world feel that what they write can be used against them."
Sir Kim Darroch's private cables to the British government were leaked, remember.
Mr Tugendhat says the ambassador had done the "right thing" in his communications and "should be supported".
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Dave Penman, general secretary of the FDA civil service union, says Sir Kim Darroch was "placed in an impossible situation".
"Firstly, by the leaking of confidential communiques and then by the failure of Boris Johnson and his supporters to provide unequivocal support," he says.
"As a loyal public servant, he has, as he always did, put his country first. Can we honestly say that about those who took to the airways and equivocated?"
He says a "disturbing pattern" was emerging - "here the brightest and best civil servants are being forced out of their jobs due to relentless attacks from politicians and some sections of the media."
He says it is the job of ministers and of the prime minister "to defend their civil servants from this kind of abuse".
"But time and time again, they are choosing to stay silent and as a result they have created an environment where the targeting of individuals or undermining of the service as a whole has become routine."
The Washington Post
There hasn't been much reaction through yet from the United States on Sir Kim Darroch's resignation.
But the Washington Post is reporting some remarks from Vice President Pence's chief of staff Marc Short.
The newspaper reports that he told journalists in Washington: "I think the reality was that, in light of the last few days, his ability to be effective was probably limited.
“So it was probably the right course.”
BBC News ChannelCopyright: Reuters
Former defence secretary and Boris Johnson supporter Michael Fallon tells the BBC the leadership contender was not to blame for Sir Kim Darroch's departure.
"The person responsible is the person who leaked this," he says.
"Boris Johnson supports all our diplomats around the world - he did that as foreign secretary, he does that today."
He says Sir Kim's position was "already untenable" after the email leak, "long before last night's leadership debate".
The backlash against Mr Johnson is "a shabby attempt to politicise" the affair, Mr Fallon adds.
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International Trade Secretary Liam Fox was with Sir Kim Darroch in Washington on Tuesday night ahead of his resignation this morning.
"I think this case raises a number of very difficult issues," he said.
"The first is that somebody has, for whatever malicious reasons, sought to either damage the US-UK relationship or indeed Sir Kim personally.
"I don't know what the motivations would be at this stage, but it's very important that we find out who actually carried out this particularly damaging act.
"It's important that our ambassadors are able to write frankly to politicians."
He called Sir Kim a "very fine public servant".
After a busy few hours, here is what we've learnt:
- Sir Kim Darroch has resigned his role of UK ambassador in Washington, saying the email leak had made it "impossible" to continue
- Theresa May said Sir Kim's departure was "a matter of deep regret" and he had the full backing of the cabinet
- Foreign Office boss Sir Simon McDonald said the leak was "malicious"
- He told the foreign affairs committee it was the first time in his career that a head of state had refused to work with a British ambassador
- The police are involved in the investigation into the leak, Sir Simon said
- There has been criticism of leadership frontrunner Boris Johnson, who declined to support Sir Kim in the role when asked in last night's TV debate
- When asked why he was not more supportive of Sir Kim, Mr Johnson said it was "wrong to drag civil servants into the political arena"
- Fellow Tory leadership candidate and Foreign Secretary Jeremy Hunt said he "profoundly" regretted the "outrageous" leak that led to Sir Kim's departure
Mr Trump has not yet addressed the row since Sir Kim's resignation.