Business guru Sir Gerry Robinson said the Prime Minister appeared incapable of leading and had left the party in "probably an impossible position to come back from".

Sir Gerry has given £70,000 since 2001, although the last instalment came in 2005, but said he would no longer be giving the cash-strapped party his financial backing.

"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," he told BBC1's Andrew Marr Show.

"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 was among a number of high-profile party backers who spoke out against Mr Brown's leadership in the Sunday Times.

With Labour facing financial difficulties over the repayment of millions of pounds worth of loans, the desertion of backers could be disastrous.

The criticism piled further pressure on Mr Brown the day after key ally Wendy Alexander quit as Scotland's Labour leader over a donations row.