Charles Kennedy has attacked Mr Howard's Iraq stance

The Lib Dem leader went on the offensive after Michael Howard said Tony Blair should be prepared to criticise the US over events post-war.

But Mr Howard insisted that it could be "perfectly appropriate" for the UK to publicly give its view.

Former Labour minister Frank Field said he was "staggered" by the lack of planning for post-war Iraq.

10 June elections

With less than three weeks to go before the European, local and London mayoral elections, Mr Kennedy denied his own party's campaign was opportunistic.

Meanwhile, Leader of the Commons Peter Hain said Labour was ready to take on board the message of voters in the 10 June polls.

"This is the first difficult period we have really had as a government - we are now in our eighth year of power - and we will obviously have to listen to voters tell us on June 10," he told Sky's Sunday programme.

Mr Hain rejected the idea that a poor outcome would prompt the prime minister to stand down.

"I am sure Tony Blair will lead us into the next general election," he said.

"He has been the most successful prime minister the Labour Party has ever had and, I would argue, Britain has had in living memory," he said.

"I want him to continue doing the job for as long as he and the voters want him to do it."

'Legitimate'

Mr Kennedy said he thought Iraq was a "legitimate" election issue as a million protesters had not been listened to by the government.

In the Commons, he had been called "Charlie Chamberlain" by Tory MPs, he told GMTV.

"It really is chicanery and blatant opportunism of the worst kind for the Conservatives really to try and wish away the fact that they gave whole-hearted blank cheque support to the government over the war," he said.

Meanwhile Mr Howard told the BBC's Breakfast with Frost programme that he had a duty to ask questions when "things went wrong" in Iraq.

'Real difficulties'

"We do face real difficulties in Iraq, it's a very serious situation - look at the damage that's been done by what happened in Abu Ghraib. I don't think people have yet faced up to it," he said.

He said when people tried to advocate "Western values" from now on it would "provoke hollow laughter in the Arab world".

Mr Howard rejected suggestions he had been "deluged" with mail from angry members of George W Bush's Republican party criticising him for not supporting the prime minister.

"I haven't been deluged with letters of that kind," he said.

Despite his concerns, Mr Howard insisted he stood by his previous support for invading Iraq.

Oil prices

Mr Field told GMTV that the prime minister needed to "grow up" and realise that "people don't love him".

He warned of a "treble whammy" of issues that could erode support for Labour in the 10 June elections, citing mortgage rates, oil prices and industrial action.

But Mr Field rejected suggestions Mr Blair would step down before the next general election.

"I think he will lead us into the election and I think he will win it for us," he said.