The pledge is more than a quarter of the total amount given to the party in donations during the past year.

The BBC's Guto Harri reports: "This latest gift, in Labour's owns words, is enormous"

The revelation provoked a scathing reaction from Conservatives, who recently faced vigorous attacks over the large sums donated to the party by treasurer Michael Ashcroft.

Labour said Lord Sainsbury's donation would be presented in a single cheque within the next calendar year.

It also disclosed the total given to the party for the year ending on 31 December 1998 amounted to £7.92m. Labour's total income during the same period was £21.09m.

Lord Sainsbury: Wants his donations to be publicly known

In a statement, Lord Sainsbury said: "As a minister I am more than ready for it to be a matter of public record how much money I give to the Labour Party, and I am therefore announcing today that I will be making a gift of £2m to the party."

The minister, who is part of the family behind the supermarket chain bearing their name, has previously caused controversy for the government over his alleged links to companies involved in promoting genetically-modified crops.

The BBC's John Kampfner reports: "Labour have denied any impropriety or conflict of interest"

His statement added: "I am proud to serve in a New Labour government with its long-term programme to rebuild Britain, and equip the country to face the challenges of the future, and proud to support the Labour Party financially alongside the many other people who contribute generously to it."

Party sources said they were not embarrassed by the size of the gift.

"It's an enormous and generous gift and we would be very surprised if it isn't among the biggest single donations we have ever received."

They insisted it would not affect Lord Sainsbury's ministerial career.

Andrew Lansley: Says Lord Sainsbury should decide if he is a minister or donor

"You know the prime minister, if Lord Sainsbury was not up to his job he would be out and that's that."

But Tory vice chairman Andrew Lansley said: "The scale of Lord Sainsbury's funding of the Labour Party would not be a problem were it not for the fact that he wishes to have influence over Labour Party policy and the policy of the government.

"We have already seen how his known views on GM foods appeared to hold the government back from speedily addressing public concerns.

"This massive donation raises questions over Lord Sainsbury's position. Either he should be involved in the Labour Party through funding it, or be a minister and involved in policy making, but he should not do both."