Mr Brown has been having a tough time after a year in office
Labour is doomed under Gordon Brown's leadership, former party donor Sir Gerry Robinson has said.
Sir Gerry, a businessman who last gave money to Labour in 2005, told BBC News Mr Brown had put it in "probably an impossible position to come back from".
He was one of four Labour backers to tell a newspaper they were reluctant to fund Labour with Mr Brown at the helm.
Health Secretary Alan Johnson hit back at Mr Brown's critics, insisting the prime minister had had a "good year".
But there were few signs of celebration as Mr Brown marked his first 12 months in Downing Street on Friday.
Labour was still reeling from a second by-election drubbing in a month, after coming fifth, behind the BNP and the Greens, in Tory-held Henley.
Mr Brown's woes were compounded on Saturday when key ally Wendy Alexander quit as Scotland's Labour leader over a donations row.
The party is also facing a financial crisis, with the prospect of repaying millions of pounds in loans made to it by wealthy individuals when Tony Blair was in charge.
The party - which files its annual accounts next week - is relying on support from the big trade unions as it seeks fresh donors.
Sir Gerry Robinson: "It doesn't feel right"
But Sir Gerry, who has given £70,000 to Labour since 2001, said he would not be contributing any more funds until there was a change of leader.
He told BBC One's Andrew Marr show: "It is very straightforward. I think Labour are looking in a lot of trouble and I think Brown is showing all the signs of not being a capable leader.
"These crises don't happen accidentally. It's not just an issue of timing; it's an issue of either getting it right or not getting it right. And it doesn't feel right.
"In the end, politics like everything is about leadership and leadership is about a capacity to make us feel we are doing the right thing at the right time and making everybody feel good - that the country's in good shape.
"If you haven't got that capacity then I wouldn't back it frankly, And I don't think Gordon has that capacity.
"It is a very, very difficult position to come back from. In fact, probably an impossible position to come back from."
Sir Gerry is a former chairman of Granada television and drinks giant Allied Domecq, who last year starred in the BBC TV series 'Can Gerry Robinson Fix The NHS?'.
He was among four party backers to speak out against Mr Brown's leadership in the Sunday Times.
They included businessman Sir Maurice Hitter, who told the newspaper Mr Brown did not have the "charisma" to lead the party and Sir Christopher Ondaatje, who attacked Mr Brown over his handling of the Northern Rock crisis.
Another donor, Everton chairman Bill Kenwright, told the newspaper the party had suffered an "energy bypass" and the leadership needed a "quantum change".
On Friday, Lord Levy, Mr Blair's former chief fundraiser, also suggested the party should change its leader, in an interview with BBC Two's Newsnight programme.
But Alan Johnson insisted Mr Brown was still the right man to lead the country.
He told the BBC's Politics Show: "What the public want is someone who has the courage, the decency, the integrity, the intelligence to solve the major problems we are facing all over the world.
"In Gordon we have got that. He will never ever try to be some sort of showbiz personality. That's not Gordon and his personality should not change.
"But increasingly as the focus comes on policy and integrity and dedication and decency, I think people will like what they see," said Mr Johnson, who has been tipped as a future leader if Mr Brown steps down.
"In the analogy of Wimbledon, Gordon is not interested in strutting his stuff on the show courts; he is quite happy to do it on the outer courts which makes him quite unique for a politician.
"But in terms of what he is achieving, in very difficult circumstances, I think it has been a good year."