Labour has received a £2 million donation from Lord Sainsbury.
The peer's money will be used during the forthcoming general election campaign, the party said.
The billionaire Science Minister said: "I am pleased to be able to make this contribution to the general election funds of the Labour Party because I believe that they are the only party that has the vision to deliver both economic prosperity and better public services for Britain."
Matt Carter, Labour Party general secretary, said the money was a "fantastic gift", enabling the party to fight and win the election "whenever that comes".
It is widely expected to take place on May 5, to coincide with local elections.
"Through the generosity of David Sainsbury and other Labour supporters, Labour will be campaigning hard to take Britain forward, and to stop the Tories taking us back," Mr Carter added.
It is estimated 64-year-old Lord Sainsbury has given more than £10m to Labour since 1999.
He donated £2.5m to the party in 2003.
Forbes' rich list says the peer is worth $4.3 billion.