The British Government must act against Sinn Fein after police raids at the republican party's offices at Stormont, Northern Ireland's First Minister David Trimble has said.

Mr Trimble, who is the Ulster Unionist leader, said he did not see the Northern Ireland Assembly surviving in its present form unless action was taken.

Sinn Fein's offices were searched as part of a major police investigation into intelligence gathering by republicans. A large quantity of documents was seized.

Three men and a woman were detained in the operation, which included searches in north and west Belfast.

Gerry Adams: Complaint to government over raids

Among those held was Sinn Fein's head of administration, Denis Donaldson, and a former junior employee of the Northern Ireland Office.

It is believed he had access to correspondence belonging to the Northern Ireland Secretary, John Reid, including some documents from Downing Street.

David Trimble said the matter would have "grave implications" for the future of the power-sharing government at Stormont.

He said he suspected IRA intelligence was "directed at the upper echelons of the government, having penetrated the Northern Ireland Office".

He ruled out an imminent withdrawal of his ministers from the power-sharing executive but warned "the primary responsibility is with John Reid and Tony Blair to act now".

'Political semtex'

Two weeks ago, he gave Sinn Fein an 18 January deadline for proving the republican movement was committed to peace, or his ministers would quit the executive, after allegations of continuing IRA activity.

Northern Ireland Secretary John Reid said there would have to be "tough talking" in Downing Street and elsewhere over the next few weeks.

It's certainly the biggest crisis since the signing of the Good Friday Agreement

Denis Murray BBC NI correspondent

"The investigation, of which I was informed about, is into events which took place some time before 24 July."

That was the date in July when Dr Reid warned Sinn Fein that action would be taken if IRA activity did not cease.

Dr Reid added: "I'm trying to draw a line in the sand and warn everybody that whatever had happened in the past, this was not going to be tolerated in the future."

The BBC's Northern Ireland correspondent, Denis Murray said the fallout for the political process could be "enormous".

"This isn't just political dynamite, it's political semtex," he said.

"It's certainly the biggest crisis since the signing of the Good Friday Agreement."

'Anti-peace process'

Sinn Fein President Gerry Adams said he had complained to the British Government about the searches.

He accused the police service of being "anti-peace process, anti-Sinn Fein and anti-democratic".

Sinn Fein head of administration Denis Donaldson was arrested.

The raids began at about 0500 BST on Friday.

The Police Service of Northern Ireland said the search at Stormont was "confined to one desk and immediate surrounding area".

They also said a number of computer discs were "seized for further examination and an officer of the assembly was present throughout search".

Irish Foreign Minister Brian Cowen warned people not to rush to make judgement.

Mr Cowen said the issue would be raised at a meeting of the British and Irish prime minister's on Wednesday.

Security sources have said the searches and seizures were not connected to the investigation into the theft of intelligence files at Castlereagh police station on 17 March.

A Sinn Fein spokesman said that homes of "community activists" in west Belfast had also been raided on Friday morning.

The spokesman said documents related to policing had been seized.

On Friday afternoon, about 60 people held a protest at the headquarters of the Policing Board in Belfast against the raid at Sinn Fein's Stormont office.

Stormont, in east Belfast, houses the Northern Ireland Assembly. Sinn Fein has about 18 offices there.