Police investigating IRA intelligence gathering have arrested a civil servant who had access to the office of the former first minister, David Trimble.

The man was arrested after a security operation in Belfast which included searches in offices at Castle Buildings at Stormont.

It is understood the man helping police with inquiries had access to ministers' diaries.

Mr Trimble said the arrest was "a most concerning and serious development and appears to have huge implications for the political process".

David Trimble said arrest could have serious political implications

He said the arrested man had access to "sensitive" information.

Prime Minister Tony Blair's spokesman said the allegations were "serious and disturbing".

The arrested man also had access to the office of SDLP leader Mark Durkan, the former deputy first minister at Stormont.

Mr Durkan said the man worked in his private office for a number of months.

"I would caution against any rush to judgment in this matter and would ask that the media respect the privacy of the civil servant concerned," he said.

The development came a day after Northern Ireland Secretary Paul Murphy announced that the government was to convene all-party talks aimed at reinstating the political institutions.

Northern Ireland's power-sharing executive was suspended in mid-October amid allegations of IRA spying within the Northern Ireland Office.

'Undermines trust'

Four people, including Denis Donaldson, head of Sinn Fein's administration at the Northern Ireland Assembly, were charged last month after police seized hundreds of documents in raids on republican homes.

Mr Blair refused to give in to Mr Trimble's demands to have the two Sinn Fein ministers expelled from the executive.

A spokesman for the Police Service of Northern Ireland said they could not comment on specific details about the latest arrest but confirmed a man was detained late on Wednesday afternoon.

Speaking outside Downing Street, the Northern Ireland secretary said the latest arrest "simply undermines confidence and trust in the peace process".

He described the development as "very serious" and said he had spoken to Mark Durkan and David Trimble.

NI Secretary Paul Murphy: "Serious development"

Sinn Fein's Conor Murphy said there was now a growing belief that there was a witch-hunt against Catholic civil servants.

"Many people are now considering there is some form of a McCarthy-type witch hunt against Catholic civil servants who happen to live in nationalist working class areas," he said.

Loyalists warned

The prime minister's spokesman said it was important that the police continued to investigate the allegations.

The development underlined the prime minister's belief that the issue of trust needed to be addressed, he added.

Meanwhile, the BBC has learned that a number of loyalists were visited by the police on Thursday morning to inform them that their personal details were contained in documents seized as part of the Stormont spy inquiry.

Davy Adams: "I am angry I wasn't informed"

Names, addresses and other information about the loyalists in Belfast and other areas were allegedly on a document recovered in October as part of the investigation into alleged IRA spying.

A police spokeswoman said: "This notification process is part of an ongoing operation to inform people of the situation and brief them on improvements to their personal security."

One of those named on a list was David Adams, a former Ulster Democratic Party spokesman.

He said he was "very angry and concerned" that the police had not informed him and that he had had to hear about the list from the BBC.

After the first seizures, the police began alerting about 2,000 prison staff in Northern Ireland that their personal details were in the hands of republicans.

Later on Thursday, the Irish Foreign Minister, Brian Cowen, is due to meet delegations from the Ulster Unionist Party and the Women's Coalition in Belfast on Thursday as the government prepares to review the implementation of the Agreement with the parties.

Mr Cowen is to meet Sinn Fein, Progressive Unionist Party and Alliance Party leaders on Friday.