|PIC1|MI5 has taken the unusual step of clarifying its stance about the government proposal to increase the period terror suspects can be held without charge to 42 days after press reports that it opposed the idea.

Jonathan Evans, director general of the agency, said the security service had "not sought to comment publicly or privately" on pre-charge detention time limits.

"Since the security service is neither a prosecuting authority nor responsible for criminal investigations, we are not, and never have been, the appropriate body to advise the government on pre-charge detention time limits," he said in a statement on the MI5 website.

"We have not, therefore, sought to comment publicly or privately on the current proposals, except to say that we recognise the challenge posed for the police service by the increasingly complex and international character of some recent terrorist cases."

The statement comes after a weekend of media reports which claimed MI5 did not support an increase in the detention time limit from 28 days to 42.

Parliament votes on the proposal on Wednesday, with many Labour MPs widely reported to be considering voting against.

Up to 30 Labour backbenchers are considering joining opposition parties in opposing the plan, on the grounds it will infringe civil liberties, the BBC said.

Gordon Brown, whose government says 42 days will occasionally be necessary to deal with increasingly complex terror plots, has turned the ballot into a crunch vote after saying he will not back down.

The vote is predicted to be so close that Foreign Secretary David Miliband was forced to scrap planned meetings with Israeli leaders in Jerusalem on Tuesday and return to London.

Brown's poll ratings are at an all-time low after the loss of the formerly safe Crewe and Nantwich parliamentary seat and a drubbing in local elections, fuelled by a slowing economy, rising oil prices and a series of unpopular decisions including the planned abolition of the 10p tax rate.

Defeat in the security vote would seriously erode the prime minister's authority still further.

Attempts by his predecessor Tony Blair to extend detention without charge to 90 days in 2005 ended in his first Commons defeat as prime minister.