Work and Pensions Secretary Peter Hain has resigned from the Cabinet after donations to his campaign to be Labour's deputy leader were referred to police.
There has been reaction from across the political spectrum - here is a selection of the comments made so far:
ALAN JOHNSON, Health secretary
I am really sad. Peter is an excellent secretary of state.
I have worked with him for many years. We were in the Communication Workers Union together - he was head of research when I was general secretary.
I can understand why he's done this now it's been referred to the police, and I think it shows great credit on Peter that he believes that now it's got to that stage, he cannot carry on in a Cabinet position.
But I am really sad, because it's a big loss for us.
DAVID CAMERON, Conservative leader
It's the right decision, but it shouldn't have happened in this way.
I said some time ago the prime minister should have said to Peter Hain, 'You've got to give a convincing explanation of your situation or you can't stay in the cabinet.'
Instead we've had a long delay where one of the most important departments in government hasn't been led properly, and I think that was wrong.
The Department for Work and Pensions is one of the most important jobs in government. It is one of the highest-spending departments, responsible for pensions, responsible for benefits and welfare reform.
It is a huge department and a very big hole will be left by Mr Hain's departure, which the prime minister will have to fill. The prime minister should not have allowed this to go on for so long.
CHRIS GRAYLING, Shadow work and pensions secretary
Peter Hain's resignation was inevitable and the right thing to do, given the Electoral Commission's decision.
What is important now is for Gordon Brown to take rapid action to restore effective leadership to a department that has clearly been distracted by the events of the last few weeks.
DAVID DAVIS, Shadow home secretary
When you go to a police investigation, it must be very time-consuming, very worrying - and he's got, or had until a moment ago, a very serious job at DWP [the Department for Work and Pensions] and a very serious job in Wales.
It would have been probably impossible for him to do his Cabinet role at the same time as dealing with this, so I can't say I'm surprised, and we'll have to see what comes out of it.
He wants to clear his name - well, let's see what comes out in the next couple of months.
HARRIET HARMAN, Labour's deputy leader
I regard the work that my Rt Hon friend has been doing in Work and Pensions - on improving the income in retirement of pensioners, ensuring that more people come off benefit and get into work - has been important work for people in this country.
I personally regard him as an excellent colleague and good friend.
DANNY ALEXANDER, Lib Dems' work and pensions spokesman
The transition from Blair to Brown feels increasingly like the transition from Thatcher to Major.
We have had the return of government incompetence, economic turmoil and political sleaze.
We all remember John Major clinging on to ministerial colleagues, only to lose them in the end, and now Gordon Brown is doing the same.
His colleagues are being caught out by allegations of self-interest and complacency.
The difference is that it took Major almost two years to lose his first Cabinet colleague. Gordon Brown has lost Peter Hain after just six months.
CLIVE PHILLIPS, President of Hain's constituency Labour Party
I am very saddened. I think he has been hounded out. He came clean as soon as he found out about the undeclared donations.
I saw him on Saturday night and he was so upbeat and enjoying himself. I had not seen him like that for days, and I thought he was back to his old self.
It has come as a real shock.
ELFYN LLWYD, Plaid Cymru's leader at Westminster
I think he's done the honourable thing.
I believe that his position was untenable when this Progressive Policy Forum came to light.
We still don't know exactly what it is because nobody's been able to offer a sensible explanation for what that forum did or does.
PAUL FLYNN, Labour MP for Newport West
I believe this is the result of a nasty, vindictive witch-hunt against a decent, honourable politician.
I believe this has been gone over in great detail, and there's not a speck of sleaze found in the affair.
What he's guilty of is being dilatory in making what are perfectly legal contributions.
RHODRI MORGAN, First minister of Wales
It is a personal calamity. There is a great deal on the credit side of somebody who is an outstandingly gifted politician.
I feel enormous sympathy for him because when this happens, of course, well - I am just trying to imagine what he must feel like tonight.
I think we all ought to remember really that politics can be rough, it can be brutal and, you know, you just have to feel sympathy when this kind of thing happens.
MICHAEL LEAHY, General secretary of the Community union
Peter Hain made a major contribution to this Labour government, not least in his time as Northern Ireland secretary and as secretary of state for work and pensions.
His work with Community to bring an end to the misery affecting the 130,000 people who lost their expected occupational pension due to employer insolvency is a legacy of which he can be proud.
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