A fresh £2.5m donation to Labour coffers from Science Minister Lord Sainsbury has been criticised by a member of the party's national executive.
Labour has a £6m overdraft
Mark Seddon branded the donation a kind of "corruption" of the political process and urged the party to return the billionaire's gift.
The Labour peer's donation brings his contributions to the party since 1999 to £8.5m.
In a statement, Lord Sainsbury said: "In our democracy political parties have to raise funds to campaign and put their policies to the electorate.
This very generous donation from Lord Sainsbury is fantastic news for our
"As a proud supporter of the Labour Party I am happy to be in a position where I can make a contribution to its ongoing work."
That explanation failed to impress Mr Seddon.
He told BBC Radio 4's Today programme: "In any other country I think a government minister donating such vast amounts of money and effectively buying a political party would be seen for what it is, a form of corruption of the political process.
"This was a criticism of the Conservatives when they were in government and increasingly people are looking in at the political parties and saying 'Why don't they have more members?'"
Mr Seddon said accepting the money was "quite extraordinary" when the party was finding it difficult to get trade union funding.
"It should be sent back straight away," he added.
Lord Sainsbury has given a total of £8.5m to Labour
Conservative chairman Theresa May said Lord Sainsbury's position as a minister appointed by Tony Blair and whose decisions could have commercial consequences raised real questions about the gift.
"What all political parties need to do is to try to encourage a larger number of people to give small donations so parties are not reliant on large donations from individuals of this sort," Mrs May told Today.
Before the 2001 election, donations to the Tories from philanthropist Sir John Paul Getty and betting tycoon Stuart Wheeler totalled almost £8m.
But Mrs May said her party was now making progress on receiving more smaller gifts - and suggested Labour's failures meant its own members would not do the same.
A Labour spokesman said there was no particular significance in the timing of the donation from Lord Sainsbury, the former chairman of the supermarket chain founded by his great-grandfather.
But Labour general secretary David Triesman said: "We are grateful to everyone who contributes whatever they can to the Labour Party.
"This very generous donation from Lord Sainsbury is fantastic news for our party. We remain hugely grateful to Lord Sainsbury for his continued steadfast
Last week, Mr Triesman cast doubt on claims that disquiet about the Iraqi conflict had caused grassroots members to leave the party in droves.
While a total of 4,900 people had resigned their membership in the last year, there had been a net figure increase of just under 9,300, he said.
Mr Triesman said "a substantial majority" of party members had come round to supporting military action.
The party has a £6m overdraft and a £4.5m mortgage on its London headquarters.
Lord Sainsbury has been a minister since 1998.
His other large charitable donations reflect his interests in mental health, technical education and Third World development through his charitable trust, the Gatsby Foundation.