The contest for the British Labour leadership is getting personal with David Miliband about to make his strongest criticism of his younger brother Ed as the contest reaches a critical stage.

The former Foreign Secretary was to suggest his brother is pandering to Labour’s core vote rather than reaching out to the middle classes and that his strategy will keep the party in opposition rather than return it to power.

David Miliband was to set out clear dividing lines between him and his brother, seen as the two front-runners in the race to succeed former Prime Minister Gordon Brown. The result of balloting will be announced on September 25.

The shadow Foreign Secretary was to say that Labour must be an alternative government as well as an effective Opposition. “Opposition is necessary but insufficient. At worst it can take us back into our comfort zone – and our pantomime role in politics. We need not just to oppose this Government. We need to defeat them.”

David Miliband insisted he is the best candidate to prevent the Government painting his party as Old Labour. “We need to break out of the mould the Coalition is trying to put us in,” he was to say. “Their attacks on our record are only the first phase of their campaign. They seek to make Labour irrelevant. I will not let them.”

He was to say Labour’s leadership contest has spent “a lot of time looking inwards and backwards, when we need to look outwards and forwards”. He is pledging to look “outside our tent” and talk about the change that Britain, rather than the party, needs.  Calling on Labour to shift to the centre ground, he was to say: “We must look forwards for new ideas and outwards for a new coalition of voters.

“We are pigeonholed as spendthrift when we need to be prudent; we are seen as accreting power to the state when in fact our mission is to empower individuals, communities and businesses; and we are seen as the establishment when we need to be the insurgents.”  Yesterday he won the support of his 100th Labour MP.

Ed Miliband, the shadow Energy and Climate Change Secretary, moved to head off his brother’s criticism by insisting he is “the moderniser”. He said that, since 1997, Labour had lost 1.7 million middle-class voters. But he argued that Labour had lost a total of five million voters overall. Ed Miliband said: “I am the only candidate who can reach out to those whom we have lost. To those middle-class voters who have switched to the Lib Dems I say look at how Labour is changing.”