Analysis By Nick Assinder Political Correspondent, BBC News website

So the Conservatives have got their Tony Blair figure. The challenge for their new leader, 39-year-old David Cameron, is to deliver the same sort of transformation of his party that the prime minister managed with his a decade ago.

Brown plans to face off Cameron

Mr Blair's success in first recreating the Labour Party and then in winning the 1997 General Election brought an end to the Conservatives' 18 years in power.

So overwhelming was the defeat suffered, that Mrs Thatcher's party was pitched into protracted internal battles, two further election defeats and now four leaders during what has been thus far, eight years in opposition.

Mr Cameron, two years younger than Mr Blair when he took over the Labour Party in 1994, has dismissed suggestions he is "Tory Blair" - but he will be hoping to prove as electorally successful as the man he will now be facing head-on each week in the boisterous weekly prime minister's questions.

With his even more youthful shadow chancellor, George Osborne, the new Tory leader ran a campaign based on offering optimism and a clear break from the past.

With Tony Blair due to stand down before the next election, which is due by 2010, they have targeted his likely successor, Chancellor Gordon Brown, as "past his sell-by date" and as a "roadblock" to the centre-ground reforms which have brought Labour three successive landslides.

The question now is just how Tony Blair's party respond to the new challenge?

We got the first hints even before Mr Cameron was confirmed in post.

They came from Chancellor Gordon Brown who made it pretty plain during a BBC radio interview that he expects to be the man tackling Mr Cameron from the dispatch box.

So that, then, might be one of the more obvious reactions to Mr Cameron - a change in Labour leader.

And, if Mr Brown's apparent certainty about his own position is any guide, that may well come relatively quickly - and certainly long before the general election.

Old Labour

The next reaction to Mr Cameron will, apparently, be to speed up the process of reform started by Tony Blair.

Asked if he would run a Blairite administration, the chancellor declared: "exactly".

Cameron aims to transform Tories

But he went on to suggest the pace of reform would have to intensify rather than slow down once he is leader.

That may not delight some of his more traditionally-minded Labour backbench supporters, but it is a message he has now been delivering for many months, most notably at the annual Labour conference in September.

Whether it exactly matches what he does in practice may be another matter. After all, there have been any number of Cabinet rows suggesting Mr Brown has a different idea of what constitutes a reforming agenda.

Nothing new

But what about the issue of age. Won't it be hard for him to combat somebody who will be in his mid forties at the time of the next election when Mr Brown will be nearer 60?

Still not fazed, the chancellor pointed out he had a two year old child which kept him feeling pretty young. And, in any case, the defeated former Tory leader William Hague had been even younger than Mr Cameron.

And, as for questions over how Prime Minister Brown might tackle the new Tory leader's policies, he declared that on the key issue of taxation and public spending, Mr Cameron was offering nothing new.

His call to keep spending below the level of growth was simply a re-branding of the old Tory programme of cutting public services.

So, in essence, the battle will be between a Labour Party that promises investment in public services and a Tory party that would cut those services, he said.

That sounds terribly familiar and it is probably fair to suggest that, once the Brown-Cameron test of strength finally kicks off in earnest, things might look dramatically different.