Brown said his governing, centre-left Labour Party would create "more middle class jobs than ever before" if it overturns the opinion polls and wins the general election due by June.
Brown has previously sniped at main opposition centre-right Conservative leader David Cameron for his privileged education and background.
That led the Tories to accuse him of fighting an old-fashioned, left-wing class war in a bid to shore up Labour's traditional vote -- in the face of a looming defeat.
In a major speech to centre-left activists, Brown was to say: "Social mobility will be our theme for the coming election and the coming parliamentary term.
"A fair society is one where everyone who works hard and plays by the rules has a chance to fulfil their dreams whether that's owning a bigger house, taking a holiday abroad, buying a new car or starting a small business.
"This is the next project for New Labour... The coming decade will provide the UK with more middle class jobs than ever before."
Brown wrote in The Guardian newspaper that he wanted to see "an expanded middle class, not a squeezed middle class".
"Opportunity and reward cannot be hoarded at the top, and it is not enough to protect people at the bottom," he said.
"Our values demand a genuine meritocracy for all British people, and I want to set out how in the coming decade we can unleash the biggest wave of social mobility since the Second World War -- to spread opportunity across society and to realise the aspirations of all those on middle and modest incomes."
Labour's manifesto would be pitched at "anyone who wants to get a home, start a business, build a career or save for their children's future," the premier wrote.
The Conservatives blasted Brown's latest move.
"One minute Gordon Brown's a class warrior, the next he is a friend of middle Britain," said Treasury spokesman Philip Hammond.
"Middle Britain won't forget that it was Gordon Brown who destroyed their pensions, increased their taxes and crippled social mobility.
"The idea that a man who has spent his whole career at war with the middle classes can be their champion is laughable."
Meanwhile Brown's de facto deputy, Business Secretary Peter Mandelson, said veering to the left for the core vote was "not a winning strategy, as every member of the government knows".
"The election is not won by us, nor is it yet lost," he told The Daily Telegraph newspaper.
He also said the new 50 per cent top income tax rate for those earning more than 150,000 pounds ($244,000, 170,000 euros) a year should be abandoned as soon as the public finances allow it.
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