GORDON Brown's make-or-break conference in Manchester began today with a double boost for the Prime Minister.

The man often seen as his main rival for the Labour leadership, Foreign Secretary David Miliband, issued a call for party unity. And best-selling Harry Potter author JK Rowling announced that she was donating £1m to the party.

She hailed Labour's record on child poverty and launched a broadside at Conservative plans for tax breaks for married couples.

Mr Brown will hope that the developments set the scene for a five-day gathering in which he can rally the party faithful and turn Labour's fortunes around after months of plummeting poll ratings, a backbench rebellion and a ministerial resignation.

But the latest poll released today made grim reading for the Prime Minister, with YouGov putting Labour on just 24 per cent, 20 points behind the Tories on 44 per cent, and Liberal Democrats closing in on 20 per cent.

Ms Rowling's £1million donation will ease financial difficulties for Labour, which has debts of £17.8 million.

But perhaps more useful to the party will be her ringing endorsement of Mr Brown's approach to child poverty and her stinging attack on the Tories.

"I believe that poor and vulnerable families will fare much better under the Labour Party than they would under a Cameron-led Conservative Party," said the multi-million selling author, who was a cash-strapped single mother when she wrote her first story about boy wizard Harry Potter.

"Gordon Brown has consistently prioritised and introduced measures that will save as many children as possible from a life lacking in opportunity or choice. 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."

Generous

Mr Brown 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."

Mr Miliband today urged Labour activists to 'pull together' behind Mr Brown and not be defeatist, telling them that the conference provided an opportunity to 'put a strong, determined, clear, unified face before the public'.

He said: "It is time for the party to come together.

"I've made it clear I don't think it's the time for a leadership election. It's time to address the fundamental challenges - that's why it's the time to pull together."

He added: "When I look at Gordon, (Chancellor) Alistair (Darling), our team of ministers, we've got people who haven't lost the hunger for government - and when I look at our party members, I know they know the difference between government and opposition.

"They haven't forgotten the dark days of opposition and the damage it does to the country."

Meanwhile, 20 Labour MPs sent a letter to all their parliamentary colleagues urging them to 'stand up and be counted' by joining a loyalist campaign launched by former Deputy Prime Minister John Prescott to secure a fourth term in power for the party.

As well as Mr Cruddas, signatories of today’s letter include Treasury minister Angela Eagle, local government minister John Healey and environment minister Joan Ruddock and parliamentary private secretaries Roberta Blackman-Woods, Andrew Gwynne, Kerry McCarthy, Chris Bryant, Rob Marris, Dave Anderson and Helen Jones, as well as former ministers Andrew Smith, Alan Whitehead, Don Touhig. Both Mr Healey and Mr Touhig have served as PPS to Mr Brown.

Also signing are Karen Buck, Martin Salter, Kevan Jones and Frank Doran as well as sometime backbench rebels John Grogan and Roger Berry.

Meanwhile, another Cabinet minister often tipped as a possible future Labour leader today ruled himself out of the job and hinted that he would back Mr Miliband as Mr Brown's successor.

Health Secretary Alan Johnson said that Mr Brown was the right man to lead the party 'at the moment' because of his economic experience, and urged rebels calling for a leadership contest to 'shut up'.

Mr Brown was facing a challenge to his authority this weekend as delegates decide whether to allow a debate on union calls for a windfall tax on utility companies, rejected by the PM when he unveiled a package of energy efficiency measures earlier this month.

And he was also facing criticism outside the Manchester Central convention complex, with thousands of people joining a Stop the War march demanding the withdrawal of British troops from Iraq and Afghanistan.