Supporters of the Premier are reported to have said the South Shields MP and Foreign Secretary behaved "disgracefully and disloyally" in writing the article in the Guardian – and refusing to rule out a leadership challenge – and that his ego had "clouded his judgement".

But last night, a senior Labour Party source told the Gazette that Mr Miliband, 43, wasn't intending to spark a leadership challenge, but instead stand up to Harriet Harman, deputy leader of the Labour Party.

She said: "When he heard Harriet Harman had allegedly said 'My time has come', he couldn't let it just slip by.

"He wanted to make sure his hat was well and truly in the ring, and show that he was a real contender should Gordon Brown go.

"But he's certainly not going to be the one to force Brown out. David just isn't like that."

The debate was sparked by the 1,000-word article in which Mr Miliband set out his vision for Labour's future.

Controversially, he didn't mention the beleaguered Prime Minister. He later discussed the issue at a press conference, which was meant to be about UK relations with Italy – but ducked the question of whether he would rule himself out of a future leadership contest.

He said: "I'm not campaigning for anything other than a successful Labour government. The truth is we had a bad result last week and I was frustrated by the fatalism in the media and in the Labour Party.

"You might insist that personality comes before issue and argument, I don't believe that."

Former minister Denis MacShane called on fellow Labour MPs to "follow Miliband's leadership" in turning their fire on the Tories.

Mr MacShane described Mr Miliband's article as "a sharp and welcome contrast to the self-indulgent, defeatist briefings by ministers in the last few days".

In a Gazette Internet vote yesterday 82 per cent of readers said Mr Miliband was ready to be Labour Party leader.