David Miliband ratcheted up the pressure on Gordon Brown today by refusing to rule future out ambitions for the leadership and repeating his belief that the party was not making a strong enough case against the Conservatives.

In a relaxed performance standing next to the visiting Italian Foreign Minister, the Foreign Secretary did little to calm talk in Westminster that he may be preparing the ground for a campaign to replace Gordon Brown.

Speculation about Mr Miliband's ambitions for the leadership intensified this week after he appeared to set out his personal manifesto for change in a Guardian article, which made no mention of Mr Brown.

Today, he refused to say how long he expected the Prime Minister would remain in the job, and stressed the importance of the role of his Cabinet colleagues.

"Gordon will lead us forward, with the rest of us having a contribution to make," he said.

Asked why he failed to mention Mr Brown in this morning's article, he said: "The debate has to be about issue and argument not personality. I might be wrong about this. You might insist personality comes before argument. I don't believe that."

However, he again made clear that he felt the party was unhappy. "The truth is that we've had a very bad result in the Glasgow East result. But I was frustrated by the sense of fatalism that has imbued parts of the media, possibly even parts of the Labour Party, and I thought it was very important to write an article that tried to say there is a Labour record which is good," he said.

"There is a very clear Labour vision for the future which offers change, significant change. And there is an emptiness about the Conservative Party that means we can get up them in a very serious way, and that is the right approach for the party."

Although giving his immediate backing to Mr Brown, Mr Miliband stepped back from saying Mr Brown would lead the party into the next election when asked if he was the "only man" to do so.

"His leadership has shown itself to be of strong value, and one which shows itself to have brought a team around it who are able to make a contribution," he said.

"Do I think Gordon can lead us into the next election and win? Yes I'm absolutely sure of this. The question is about our competition with the Conservatives, not about our competition amongst ourselves. I believe our challenges today are a challenge to David Cameron rather than Gordon Brown."

Asked to rule himself out from the leadership, he declined to do so. "How many times can I say this is about argument. Arguments about issues about a united Labour response."

Asked if it would be mad for Labour to hold another leadership primary before another election, he replied: "The Labour Party never does mad things. We’ve got to be a responsible party of government."

Yesterday, Times Online reported that Mr Miliband and Harriet Harman, Labour's Deputy Leader, are both preparing the ground for a leadership election.

It is alleged that Ms Harman was spotted in the aftermath of Labour's defeat in the Glasgow East by-election telling aides "this is my moment".

Labour's Deputy Leader, who is "minding the shop" in Downing Street this week while Mr Brown is on holiday, denied the claims.

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