In his first speech as leader to Labour's annual conference, the left-leaning Ed Miliband will implicitly criticise Brown and his predecessor Tony Blair, who won three elections with his New Labour brand of centrist, business-friendly politics.
Brown, weakened by a banking meltdown and a deep recession, quit after Labour lost the May election, leaving a record peacetime budget deficit that the Conservative-Liberal Democrat coalition government plans to tame with deep spending cuts.
When you saw the worst financial crisis in a generation, I understand your anger that Labour hadn't stood up to the old ways in the City (financial district), which said deregulation was the answer, Miliband, who was a confidant of Brown, says.
Strong trade union support helped Miliband, 40, win the Labour leadership by a wafer-thin margin on Saturday, surprisingly defeating his older brother David, the darling of the New Labour establishment.
A YouGov opinion poll for the top-selling Sun tabloid said on Tuesday Labour was more popular than Prime Minister David Cameron's Conservatives for the first time in three years. It put Labour on 40 percent and the Conservatives on 39 percent.
Miliband's speech will be scoured for signs of his stance on the unions, which hope his victory will give them a powerful ally in their fight against the coalition's cuts. They are threatening coordinated strikes against cuts they fear will lead to thousands of their members losing their jobs.
Miliband, who was Brown's energy minister, has denied he will move to the left or be a stooge of the unions.
An aide declined to say whether the speech would contain any new thinking on cutting the budget deficit.
BOOM AND BUST
In excerpts released by aides before his speech at 1315 GMT, Miliband distances himself from cherished Labour policies during 13 years in power, including deregulation of the City, London's financial district, and student tuition fees.
I understand why you felt that we were stuck in old thinking about higher and higher levels of personal debt, including tuition fees, he will say, adding:
When you saw jobs disappear and economic security undermined, I understand your anger at a Labour government that claimed it could end boom and bust.
Brown regularly boasted he had ended boom and bust during 10 years of prosperity when he was finance minister but opponents mocked him for the phrase when later the economy deteriorated.
Miliband wrote Labour's manifesto for the May election, but his speech appears to confirm his comment on Sunday that the New Labour era is over.
While praising Labour achievements, Miliband says the party had lost its ability to change during its years in power, had bought into established ways of thinking and had become the prisoner of its own certainties.
Labour should once again become a movement able to stand up for the majority in Britain and should aim to shape the centre ground of politics, Miliband says.
Blair introduced university tuition fees, paid for by taxpayer-backed loans, in 1998. Miliband proposes to replace them with a tax on graduates.
Miliband has said he may change the deficit-cutting plan put forward by former Labour finance minister Alistair Darling before the election, prompting Darling to urge him to ensure any new proposals were realistic and credible.
Darling's plan to halve the deficit in four years is far less ambitious than the coalition's proposals to virtually eliminate the deficit by 2015, when the next election is due.