John Prescott has deputised for Tony Blair at prime minister's questions, with Michael Ancram as his opposite number.

The Tory number two challenged the deputy prime minister on Blair's visit to Libya later this week on Wednesday.

Colonel Gaddafi's regime should not be trusted on the basis of its decision to renounce its weapons of mass destruction programme, the shadow foreign secretary warned.

"The prime minister should sup with a very long spoon," Ancram said.

But Prescott insisted it was "right to continue discussions" with Gaddafi.

"What we have to do is keep the peace and keep on talking at the same time as keeping a robust defence against terrorist actions."

Liberal Democrat deputy Sir Menzies Campell pressed Prescott on the road map to peace in the Middle East following the Israeli assassination of Hamas' spiritual leader.

"What steps is the government going to take to persuade Israel to comply with international law?" he asked.

Prescott said that he "deplored" the actions of Ariel Sharon's government.

However he added that the road map was "the only way forward" in the region.

"At the end of the day it is a negotiated settlement [that is needed]," he said. "We have got to keep on talking."

Prescott also appeared to deny press reports that a 15ft security barrier would be erected around the Palace of Westminster.

In a bid to embarrass the notoriously error-prone deputy prime minister, Conservative MP Peter Luff asked what Prescott would be doing to celebrate the 25th anniversary of the Plain English Campaign.

And living up to his reputation he confused parliamentary sketchwriters with "screenwriters" in his response.

Troubled history

It was not the first time Prescott had deputised for his boss.

More used to offering unofficial comment from a sedentary position at the weekly proceedings, Prescott came under the intense scrutiny of Westminster watchers waiting for an embarrassing gaffe.

He struggled in 1999, apparently being unable to distinguish between a withholding tax and the council tax.

In 2001 he told Alan Beith that policy u-turns were a sign of good government.

He also mangled key words when discussing international affairs while standing in for the prime minister. Prescott appeared to get into trouble when discussing the Taliban.

Blair meanwhile was attending a memorial service in Madrid following the recent terrorist bomb blasts in the Spanish capital.