Labour will find it hard to win the next election as its MPs are now the "underdogs", Lord Mandelson has said.
In an interview with BBC Newsnight, the business secretary said that, after 12 years in office, it was difficult for the party to retain its appeal.
He conceded Labour MPs would find it difficult to portray themselves as "the change-makers in British politics".
He did not entirely rule out becoming Labour's next leader but said "that one might be a comeback too many".
Lord Mandelson's remarks come after he insisted Prime Minister Gordon Brown's position was safe despite Labour's crushing defeat in the Norwich North by-election, which the party lost in a 16.5% swing.
In his interview with Newsnight, Lord Mandelson said Labour politicians were "the underdogs in politics now".
But he insisted the government would be "thoroughly vindicated" for the action it had taken over the economy.
"Of course it's difficult, and of course we're coming from behind," he said.
It's going to be harder for us to convince people
"But that's precisely why we've got to, after all this time in office, work even harder to demonstrate that we have the ideas and the policies but also the discipline to bring to government.
"It's going to be harder for us, in other words, to be insurgents rather than simply incumbents.
"It's going to be harder for us to convince people in the way that we need to do that we are going to continue to be, as I say, the change-makers in British politics."
Responding to a suggestion from Peter Slowe, chairman of the Labour Finance and Industry Group, that the business secretary was the only person in the party who could beat the Conservatives, Lord Mandelson said that as a peer "I can't just migrate from one chamber to another".
He said that "given all the political comebacks that I've made in my political career", becoming prime minister "might be a comeback too many".
Lord Mandelson said "tough decisions" would have to be made on public spending, but refused to use the word "cuts".
"I fully accept that in the medium term the fiscal adjustment that we are going to have to make, the rebalancing of our public finances, will be substantial," he said.
"The government has never disguised that will be the case.
"Over time, when we show this fiscal responsibility as we will need to, there will be things that we would like to do, but will have to be done slower.
"There will be things that have to be postponed and put off, and there will probably be things that we cannot do at all."
He said the Tories were now discussing "savage cuts" to public services with "almost undisguised glee".
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