LONDON (Reuters) - Britain said on Sunday it was confident the Irish Republican Army would hand in its weapons as Northern Ireland prepared for a new era of self-government after decades of sectarian and political violence.

Northern Ireland Secretary Peter Mandelson said he believed the process leading to guerrilla disarmament would begin this week now that the province had reached ``first base'' in efforts to achieve power-sharing government.

Hopes of a lasting peace rose after pro-British unionists backed forming a cabinet in which Catholics and Protestants will share power in Northern Ireland after 30 years of conflict that has cost 3,600 lives.

Placing his trust in the leadership of Sinn Fein, the IRA's political ally, Mandelson told BBC television he did not believe they would have involved the Irish Republican Army in the peace process if they did not expect a positive result.

``Gerry Adams and Martin McGuinness are two forward-looking politicians who have worked very hard to put in place this political strategy and I think they should be given the benefit of the doubt,'' Mandelson said.

``I don't think they would have asked the IRA to take part in this if they didn't know what the answer was going to be.''

The new provincial cabinet is to be set up on Monday following the decision on Saturday by the Ulster Unionist Party to join a government that will include Sinn Fein.

But UUP leader David Trimble promised to resign from the new provincial government if the IRA did not start getting rid of its weapons. He said the party council could meet again in February to review its decision to join the government.

Adams Anger

Sinn Fein President Adams responded angrily, warning that deferring a final unionist decision would fuel uncertainty, but Mandelson poured oil on troubled waters.

He said it did add another element to the province's tortuous peace deal but did not ``flatly contradict'' last year's landmark Good Friday Agreement.

``We should be careful before we become too judgmental about what David needed to do in order to manage his party,'' he said.

Nobody could predict which way the UUP vote would go. The 480-to-389 vote by the party's ruling council on Saturday was a victory for Trimble, secured in the teeth of fierce opposition.

``I have seen at first hand the sort of hostility and naked aggression he has had to go through,'' Mandelson said.

The Northern Ireland secretary helped bolster Trimble by promising that if disarmament did not follow devolved government, he would suspend the power-sharing executive.

``That was a safety net, if you like, which they were entitled to,'' he said.

He was confident he would not have to wield that stick.

``As far as I am concerned when devolution starts on Thursday, the...IRA will appoint their own representative to the decommissioning commission,'' he said.

Decommissioning, in the jargon of the peace process, means the handing in of the arsenals of guns and bombs that the IRA and their Protestant foes have amassed over the years.

By Thursday, the government in Belfast is to be fully operational. At this point the IRA is expected to name a representative to discuss the practicalities of getting rid of its weapons with the body set up to oversee disarmament.

The following week this body, headed by Canadian General John de Chastelain, is to report on its talks with paramilitaries and set out a timetable for disarming.

``I have every confidence and every faith that the IRA will appoint their representative. And I hope pretty quick progress is made,'' Mandelson said.