Downing Street has described the arrest of another person working in the Northern Ireland civil service as "serious".

Northern Ireland police investigating allegations of spying arrested a civil servant on Thursday.

The official had once worked in the private office of Ulster Unionist leader David Trimble and also had access to the office of Mark Durkan, the former deputy first minister.

Number 10 reacted with deep concern at the latest arrest.

"These kind of allegations are disturbing and they are serious," said the prime minister's spokesman.

Number 10 gave full support for the police investigation and repeated the prime minister's call for clear action from republicans to get the peace process back on track.

"We have to address these issues to allow trust to re-establish itself," said Number 10.

Both Trimble and Durkan said they were stunned by the arrest - which will raise further questions for Sinn Fein.

"We are shocked and horrified. It obviously looks like a very concerted attempted by the Provisional IRA to infiltrate the very highest office in Northern Ireland," said a spokesman for Trimble.