Former science minister and supermarket billionaire Lord Sainsbury has given the Labour party a £2m donation.
Lord Sainsbury is one of Britain's richest men
Lord Sainsbury said he was "impressed" with Gordon Brown and added Labour was the only party committed to social justice and economic prosperity.
He has given more than £16m to the Labour Party and has made a £2m loan.
One of Britain's richest men, he was one of Tony Blair's first appointments to the Lords and became a minister in 1998, resigning last year.
In a statement, he said: "I have today made a donation of £2m to the Labour Party because I believe that Labour is the only party which is committed to delivering both social justice and economic prosperity.
"This is why I joined the party in the 1960s and why I continue to support it today.
"I have been very impressed by the start that Gordon Brown has made as prime minister, and want to help ensure that he is able to continue the progress that has been made in recent years in delivering prosperity for all."
Loan 'rolled over'
Later a spokesman for Lord Sainsbury said the money was separate to the £2m loan, which is due to be repaid in September and which might now be "rolled over".
A regular Labour donor, Lord Sainsbury was among people questioned by police during the cash-for-honours investigation and was also cleared of breaching the ministerial code in April 2006 - after failing to disclose a £2m loan to the party.
He denied his resignation was anything to do with this, saying he wanted to get back to his charitable and business interests.
According to Electoral Commission figures for April to June, donations to Labour were still outstripping the Conservatives.
As of 30 June, Labour was in debt by £20.2m - a spokesman said then finances were still in a "challenging position" but there had been an "upturn".
Labour's general secretary, Peter Watt, thanked Lord Sainsbury for his "long-standing and unstinting support" and "very generous donation".