James Purnell's decision to resign as work and pensions secretary and his call for Gordon Brown to step down have prompted a number of senior Labour figures to come out in defence of the prime minister.
Meanwhile the Conservatives and Liberal Democrats say the move strengthens their case for calling a general election now.
Here are a selection of reactions to Mr Purnell's resignation.
DOWNING STREET STATEMENT
The prime minister is disappointed by the resignation of James Purnell, of which he was informed shortly before 10pm.
His focus over the coming days will be on restructuring the government on the big challenges facing the country for the future: how we guide the economy through the downturn and strengthen it for the future; how we push ahead with reform of, and investment in, our public services; and how we renew trust in our democracy and parliament.
He will continue to give his undivided attention to addressing these great challenges facing our country and putting the interests of the British people first and foremost.
FOREIGN SECRETARY DAVID MILIBAND
David Miliband said he "is not resigning" and "does not agree with his friend's decision".
JUSTICE SECRETARY JACK STRAW
A source close to the justice secretary, Jack Straw, says he is "very surprised and deeply saddened" by James Purnell's decision to leave the cabinet.
The source said Mr Straw did not support the decision, and he "remained very supportive" of the prime minister.
DEFENCE SECRETARY JOHN HUTTON
I am sorry that my good friend James Purnell has decided to resign - I think he has made the wrong decision because I firmly believe that Gordon Brown is the right man to lead our party and our country.
I urge everyone in the party to remain united behind his leadership.
CULTURE SECRETARY ANDY BURNHAM
James is a great friend of mine but I disagree with what he has done.
The country needs stability right now and Gordon Brown is the man to take us through these difficult times.
NORTHERN IRELAND SECRETARY SHAUN WOODWARD
It's so disappointing. I had a phone call about half an hour ago from the leader of the Labour group in my constituency.
This is someone who's been in the party for 50 years and she was almost in tears, because what she felt was what James had done this evening wasn't actually anything other than to just simply let down the people who need a Labour government. And I can't describe to you the disappointment that people feel.
CONSERVATIVE LEADER DAVID CAMERON
In a deep recession and a political crisis we need a strong united government. Instead we have a government falling apart in front of our eyes.
Britain deserves better than this. With this resignation, the argument for a general election has gone from being strong and powerful to completely unanswerable.
For the sake of the country, Gordon Brown must carry out the one final act of authority left open to him, go to the palace and call the general election we have been demanding.
SHADOW CHANCELLOR GEORGE OSBORNE
I think Gordon Brown either will not survive or his authority will be so damaged and destroyed that the country will not have the leadership it needs at a time when we're in a recession and when we face all sorts of other problems like the crisis of confidence in the political system.
So, the choice is clear - either we can go on with a paralysed government, whoever leads it, or we can have the general election and the public can be involved.
CONSERVATIVE PARTY CHAIRMAN ERIC PICKLES
The prime minister has lost all authority. He's lost the will to govern, and frankly it's time for him and his very tired government to go to the country and to face the consequences of his poor government record.
LIBERAL DEMOCRAT MP DANNY ALEXANDER
We have a rotten political system in this country that needs to be fixed. We have serious economic problems. It's impossible I think for Labour to fix those problems when they are concentrating at being at war with themselves.
They've lost touch with their values and they've now even lost touch with their ability to govern the country with any degree of organisation. So I think it's now time to move on, have a general election, and let's put someone else in charge
FORMER HOME SECRETARY DAVID BLUNKETT
If there was ever a moment for steady hearts and clear heads, this is it. Decisions must not be taken on a rollercoaster, with this drip-drip of deeply damaging action.
The most senior members of the cabinet must make their support for the prime minister unequivocally clear if the draining away of political lifeblood is not to become unstoppable. I am giving my loyalty to Gordon Brown, as I stated in my Guardian article today.
Everyone should take a deep breath, pause and examine the consequences of gross instability on the economy and well-being of the nation - as well as the simple, practical logistics as regards the Labour Party's constitution for what is now apparently being sought.
Everyone of any political experience needs to take a step back and act with maturity in dealing with what is unquestionably a crisis for the party and not an issue of personality - which some well-meaning colleagues appear to think.
FORMER DEPUTY PRIME MINISTER JOHN PRESCOTT
I've just discovered tonight that over the last few weeks, James Purnell was not planning for these crucial local and European elections - he was planning for his own leadership election which he exclusively launched in the Murdoch press.
Not so much a Blairite as a careerite.
FORMER LONDON MAYOR KEN LIVINGSTONE
I think he [the prime minister] is getting economic policy right and I really don't think a coup in the parliamentary party, excluding the union members and the party rank and file from deciding who the leader is, would do anything other than mean a landslide for the Tories.
He will face them down.
DEPUTY LEADER OF THE COMMONS CHRIS BRYANT
Gordon Brown is the right leader to take the country forward and I fully support Gordon as leader and prime minister.
We face two great challenges, the recession and the crisis of confidence in parliament. It's with Gordon Brown as prime minister that we can take the country through these challenges.
FORMER MINISTER PETER KILFOYLE
There are people who need the help and support of a Labour government and they are dismayed that there could be a Conservative government hastened by the actions of people like Purnell.
Purnell can believe what he likes. He has never done a proper job in his life, and it shows through in the judgment he has made and it is a woeful misjudgment.
People want strong leadership at a time of difficulty for the country and he has manifestly failed to recognise that.