Robin Cook and Eric Forth have battled over pensions and the falling value of the stock market.

With Tony Blair at the G8 summit in Canada, Cook, the Commons leader, stood in and defended the government's record on retirement funds.

Shadow leader Eric Forth began PMQs by following party leader Iain Duncan Smith's practice of throwing a number-crunching grenade at the government benches.

Forth wanted to know if Tony Blair had been right in claiming that the stock market was "massively up" on five years ago during last week's question and answer session.

"The sad truth is that pensions have lost a fifth of their value thanks to the chancellor's pensions taxes," he claimed.

Cook hit back: "The nature of financial markets is that they do go down as well as up. Stock markets have been going down over the course of the last 48 hours."

"There is no way any government can insulate their stock market from that effect. What does the right honourable gentleman want us to do? Bring in price controls on stock exchanges," he asked.

Swimming against a tide of noise from Labour MPs, Forth fought on - accusing Cook of wearing the prime minister's mantle.

"It is my first - and probably only time," said Forth referring to his role as stand-in.

"The leader may be representing the prime minister, he doesn't have to impersonate him. The fact is that the stock market is five per cent below where it was when the chancellor first hit pensions," Forth said, calling on the government to apologise to pensioners.

"I am astonished at the brass neck of the right honourable gentleman," said Cook, claiming pensioners were now better-off under Labour than they had been under the Conservatives.

"I most certainly am not going to apologise for the record of this government on pensioners," he said.

Cook's reputation for caustic wit came to the fore as he delivered a jibe at Conservative attempts to represent under-privileged people.

"Perhaps when he comes next to the House, because of his own special brand of compassionate Conservatism, he would like to tell us just how he is going to fill in the form from his leader inviting him to spend a week with vulnerable people in his constituency?" quipped Cook.

"I would immensely appreciate it if he could tell me what week it is and whether I could come in order that I can hear him explain to the vulnerable people how the last Conservative government doubled unemployment, doubled homelessness, trebled children's poverty. One week is not going to wipe out the memory of 18 years."

The pair later jousted over the lord chancellor's admission that court fines had become voluntary.

Nearly two thirds of court fines were written off, Forth told the House - and worse still 40,000 people failed to complete community service orders.

Said Cook: "Under this government, as the British crime survey shows the amount of crime has fallen by a fifth. The number of police is going up. That is in stark contrast to a Conservative government which doubled the amount of crime."

Standing in for Charles Kennedy, Paul Tyler for the Liberal Democrats asked why the government believed there was nothing wrong with the way political donations were sought.

"Shouldn't the government have a policy on this? Is it not a matter of trust," he asked questioning the donations made by the RMT and owner of Express Newspapers, Richard Desmond. "Is it not about time they did settle this issue."

"This government has got absolutely nothing to apologise for on its record on party donations. We have been open and transparent," said Cook.

Labour backbencher Gwyn Prosser asked what more had been done to crack down on people trafficking.

Cook replied there was "no greater menace than those who make large profits out of the misery and desperation of some people".

"We did make good progress at the Seville summit on getting agreement for joint action," said Cook.

Peter Lilley, a former social security secretary, returned to the issue of pension funds, asking whether the government accepted that siphoning off billions of pounds would affect payouts.

"The electorate have given their verdict on his handling of pensions," replied Cook.

Martin Salter called for greater regulation of international accounting practices in the light of the latest corporate collapse in the US of Worldcom.

"It's high time we had less bleating and whinging from the corporate world about the burden of regulation," said Salter.

"There are lessons that can be learnt from what has happened on the other side of the Atlantic," replied Cook.

Tory MP Peter Luff defended protestors against the government's new asylum centres and challenged Cook to Identify any church or organisation that was in favour of the government's plan.

"The Conservative policy for a long time was that they would detain all asylum seekers. It must be right that we provide centres that have all the services required by an asylum seeker," said Cook.

Conservative backbencher Marion Roe claimed the remarks made by education secretary, Estelle Morris, that she would not touch some schools "with a barge pole" amounted to an outrageous slur.

"I will not take any criticism from a Conservative Party that is led by a leader who will not touch any state school with a barge pole and who has sent his children to Eton," replied Cook.