Peter Mandelson declared last night that New Labour needed to "renew" itself and warned that it would lose power if it ran out of steam and fresh ideas.

Peter Mandelson declared last night that New Labour needed to "renew" itself and warned that it would lose power if it ran out of steam and fresh ideas.

The Secretary of State for Northern Ireland said Labour must learn the lessons from the party's defeat in 1951, when the radical post-war government of Clement Attlee lost the general election because it was "drifting somewhat without a clear sense of direction and purpose".

Mr Mandelson was giving the Herbert Morrison memorial lecture to commemorate his grandfather, who served as deputy prime minister and foreign secretary in the 1945-51 administration.

He said the lesson the Blair government should most heed from the period was that competent implementation of a victorious manifesto was a necessary but not sufficient condition for future success. "We have to keep thinking and rethinking our ideas. We have to hatch new policies, bold strokes and forward positions that keep us ahead of the times," he said.

Mr Mandelson's call reflects concern in Labour's high command that the party has not yet found the radical ideas sought by Mr Blair for its election manifesto. It was also a response to a warning by Philip Gould, Mr Blair's pollster, that the New Labour brand had become "contaminated".

Mr Mandelson said Labour's performance in its first term had earned the trust "to be even more ambitious in our second term". He dismissed calls to drop the New Labour label, saying: "The New Labour compass remains the best available to guide us... in a world of change." But he added that the party needed to "deepen New Labour's roots still further".