JK Rowling's personal fortune is estimated at £560m
Harry Potter author JK Rowling has donated £1m to the Labour Party, the party has announced.
Ms Rowling said she was motivated by Labour's record on child poverty and opposed a Conservative plan to give tax breaks to married couples.
Her donation will boost Gordon Brown as he tries to calm unrest among party members at Labour's annual conference.
Mr Brown said he was delighted to have the backing of "one of the world's greatest ever authors".
JK Rowling, whose fortune was estimated at £560 million in this year's Sunday Times Rich List, is known to be a personal friend of Prime Minister Gordon Brown and his wife Sarah.
The Labour party has seen a surge in funding from private individuals, who donated a total of £3.8m between April and June.
Other high-profile donors to have contributed this year include comedian Eddie Izzard, who gave almost £10,000, and Steve Lazarides, graffiti artist Banksy's dealer, who gave £120,000.
A statement from Ms Rowling, who wrote the first of her best-selling books about boy wizard Harry Potter while an impoverished single mother, pledged support for Mr Brown's long-running campaign against child-poverty.
She said: "I believe that poor and vulnerable families will fare much better under the Labour Party than they would under a Cameron-led Conservative Party.
"Gordon Brown has consistently prioritised and introduced measures that will save as many children as possible from a life lacking in opportunity or choice.
HAVE YOUR SAY
I suspect this annual get-together will be furthermore known as the Harry Potter Conference, full of make-believe, spin, wizardry, mystique and not a happy ending
Andy, Milton Keynes
"The Labour government has reversed the long-term trend in child poverty, and is one of the leading EU countries in combating child poverty.
"David Cameron's promise of tax perks for the married, on the other hand, is reminiscent of the Conservative government I experienced as a lone parent.
"It sends the message that the Conservatives still believe a childless, dual-income, but married couple is more deserving of a financial pat on the head than those struggling, as I once was, to keep their families afloat in difficult times."
'Penalty on families'
Labour's deputy leader Harriet Harman welcomed the donation during a visit to a Sure Start Children's Centre in Manchester with the Prime Minister.
A great way to kick off the conference for the PM. It may not ease his leadership worries but it shows he still has pulling power with donors, even if JK Rowling is an old family friend.
Ms Harman acknowledged that raising a daughter alone had been a struggle for Ms Rowling and said she was delighted with the reasons the author had given for backing the party.
"She's particularly concerned about the Conservatives' proposal which would put a penalty on families with young children if the parents had separated," Ms Harman added.
A Conservative spokesman said the party would not be responding to JK Rowling's comments.
Ms Rowling split from her first husband, Portuguese journalist Jorges Arantes, shortly after their daughter Jessica was born in 1993.
She wrote much of her first Harry Potter book in an Edinburgh cafe, where she found it easier to work than at her "grotty and depressing" council home.
Recognising she was a poor single parent, the cafe owners would let Ms Rowling nurse an espresso and water all day as she worked with her daughter sleeping beside her in a pram.
The author married second husband, Dr Neil Murray, in 2001 and the couple have a son, David Gordon, and daughter, Mackenzie Jean.
Her £1m donation is the first time the children's author has given cash to the Labour Party, which currently has debts totalling around £18m.
The prime minister said: "I am delighted that JK Rowling, who is one of the world's greatest ever authors, has made such a generous donation.
"I thank her for supporting the Labour Party and our values of social justice and opportunity for all."
Much of Labour's £17.9m debt was amassed to fund the 2005 general election campaign, the most expensive in the party's history.
A source indicated that JK Rowling's money would not simply be swallowed up in debt repayments, but would allow the party to continue with its work.