LONDON (Reuters) — UK Finance Minister Gordon Brown's backing for Britain to retain nuclear arms is aimed at sending a signal there will be no lurch to the left when he takes over as prime minister from Tony Blair.Brown has been trying hard to broaden his expertise and establish his prime ministerial credentials before next year when Blair is expected to step down in his favour.

Brown pledged on Wednesday to retain Britain's independent nuclear deterrent and aides said Brown backed a replacement for the Trident nuclear missile system that will be needed by 2024.

A replacement system could cost $20 billion ($37 billion), money many legislators in Brown's own Labour Party believe could be better spent on public services.

Brown's words disappointed leftwingers who hope he will abandon some of Blair's free-market "new Labour" policies and steer the party back to its socialist roots.

"I interpret it as a signal (he will) carry on the mantle of Blair," Labour legislator Ian Gibson, a member of the left-wing Campaign Group, told Reuters.

Blair, the Labour Party's most successful leader, has taken it to three successive general election victories with centrist policies, but he has pledged not to run again at the next general election, expected in 2009.

Brown has been kept waiting for years but insiders predict Blair will hand over the reins in mid-2007. He is favourite to win, even if a challenger emerges.

Former minister Clare Short, a Blair critic, said she would not support Brown in a leadership contest because he had announced his stance on nuclear weapons without consultation.

"It means a lot of people who were happy to see Brown take over as leader will now think there has got to be a contest and we're not willing to support him," she told BBC radio.

Brown is bidding to take over the leadership of a party whose popularity has been undermined by Blair's support for the US-led war in Iraq and by a series of sleaze scandals and political mishaps. It lags a resurgent Conservative Party in the polls by up to ten points.

In the past months Brown has extended his reach outside the economic arena and has tried hard to tone down his image of being a dour Scot and appeal to English voters <\m> not least by very publicly supporting England in the World Cup.

Scottish National Party leader Alex Salmond said Brown was "morphing into an Englishman" to make him more electable.

Brown has won praise for his handling of the British economy over the past nine years, steering it to uninterrupted growth with low unemployment and inflation. But he has little experience of other areas of domestic or foreign policy.

To try to remedy this, Brown has met German Chancellor Angela Merkel and held talks with political leaders in Northern Ireland this month.

With his commitment to nuclear deterrence, Brown will have reassured Britain's US allies about his reliability on defence but he has also touched a raw nerve in his own party.

Philip Cowley, a political analyst at Nottingham University said Brown's comment was an "unnecessary provocation" to as many as 100 Labour legislators who opposed replacing Trident.

During its years in the political wilderness the 1980s, the Labour Party was committed to unilateral nuclear disarmament. Former leader Neil Kinnock scrapped the pledge in the late 1980s, saying it made the party unelectable.

Patrick Dunleavy, a politics professor at the London School of Economics, said he did not believe Brown's ascent to power would bring a swing to the left.

Brown, an Atlanticist who is suspicious of Europe, would try to combine social justice, international development and aid for Africa with economic efficiency, he said.