The party has named the donors as Science Minister Lord Sainsbury and former Conservative supporter Christopher Ondaatje.
Labour's voluntary and well-ordered disclosure of the gifts on Thursday follows the criticism sparked by its initial attempts earlier this week to keep private millionaire publisher Paul Hamlyn's donation of £2m.
Mr Ondaatje, a businessman, regular donor to the arts and elder brother of author Michael Ondaatje, paid his £2m into the party's coffers within the last month. He is also to join the Labour Party.
Lord Sainsbury intends to make a gift of the same size - repeating a £2m donation he made to Labour last year.
Earlier this week Lord Hamlyn was forced to acknowledge that he was the mystery £2m benefactor after days of speculation and pressure on Labour.
The size of the gift had emerged but not the identity of the donor.
Tory attack
The Conservatives said the latest gifts showed Labour's continuing reliance on a small number of very large donors alongside trade union funding.
Commenting on his own donation, Mr Ondaatje said Labour was doing "a very good job at a difficult time and I believe they should be given every possible support".
"I say this as a disillusioned former Conservative who sees a Conservative Party divided not just on Europe but on the vital issue of the economy."
Mr Ondaatje received a CBE from the Queen last year in recognition of his support for museums and galleries.
'Proud supporter'
He met Prime Minister Tony Blair at the start of 2000 as part of a delegation from London's National Portrait Gallery, to which he gave a sizeable donation towards an extension project costing over £15m.
Lord Sainsbury, whose hefty financial backing for Labour caused opponents to question his appointment as a minister, said: "In our democracy, political parties have to raise funds to campaign and put their policies to the electorate, and as a proud supporter of the Labour Party I am happy that I am in a position where I can make a contribution."
He said he was revealing his donation now in order "to prevent any further speculation" about what donations he might be making to Labour.
A party spokesman explained they had felt it would be "right and proper" to approach both men about making their names public after Lord Hamlyn's statement on Tuesday.
He insisted Lord Sainsbury was a member of the government, not because of his donations, but because of "his considerable skills - far from profiting from his involvement with the Labour Party, he made a considerable financial sacrifice in order to be able to serve within the government".
Labour Party chairwoman Maggie Jones said she was "delighted that people of the stature of Christopher Ondaatje and Lord Sainsbury want to support the Labour Party in this way and have chosen to put their names in the public domain now."
But shadow cabinet office minister Andrew Lansley said: "This news of Labour's continuing reliance on a small number of very large donors, alongside the trades unions, for its financial backing highlights strongly the need for a broader base of party funding."
He said the need for tax relief on small personal donations was now "obvious".
Labour said 40% of its funding now comes from individual members and small donations, 30% from trade unions, 20% from large single donations and 10% from commercial activities.