Tony Blair was last night under sustained attack over the invasion of Iraq following fresh claims that the Attorney General had warned that the war could be illegal.

Tory leader Michael Howard directly accused the Prime Minister of "telling lies" over the case for war while Liberal Democrat leader Charles Kennedy warned that the General Election was becoming a referendum on the issue of "trust".

Mr Blair sought to steer the campaign agenda onto aid for the world's poorest countries with a high profile rally to mark World Poverty Day - including a video link-up with President Bill Clinton.

However the Liberal Democrats - the only one of the three main parties to oppose the war - signalled their determination to make Iraq a key issue in the closing stages of the election.

The latest controversy was ignited by a report in the Mail on Sunday which said it had seen a 13-page opinion by the Attorney General, Lord Goldsmith, in which he expressed doubts about the legality of military action under international law.

According to the newspaper, he warned there was a strong argument that it was up to the United Nations, not Mr Blair, to rule on whether Saddam Hussein was in breach of his obligations to give up his weapons of mass destruction.

Lord Goldsmith was said to have questioned whether Britain could use UN Security Council resolution 1441 - which gave Saddam a "final opportunity" to disarm - as a basis for military action.

He was said to have urged caution about launching an invasion without a second Security Council resolution while warning of the risks of relying on an earlier resolution used to eject Saddam from Kuwait.

Mr Blair has always insisted that Lord Goldsmith's advice, presented to the Cabinet on March 17 2003 on the eve of the invasion, was that military action was legal.

In a statement, the Attorney General's office again stated that he had advised that the war was lawful.

But in a bitter and highly personal attack, Mr Howard accused the Prime Minister of having "lied" and he urged voters to make the election a judgment on his character.

"He has told lies to win elections. On the one thing on which he has taken a stand in the eight years he has been Prime Minister, which is taking us to war, he didn't even tell the truth on that," Mr Howard said.

Mr Kennedy dismissed Tory attempts to exploit the issue as "laughable", saying that the Conservatives had been the "principal cheerleaders" for the invasion of Iraq.

However, he warned that unless Mr Blair published Lord Goldsmith's full legal opinion, election would become a referendum on the public trust in his decision to go to war.

"The longer it goes on the more corrosive it becomes for Tony Blair, for the Government, and for people's sense of trust in the whole political establishment."

Lord Goldsmith's office rejected calls to publish full opinion, saying it was confidential and protected by legal professional privilege. Ministers tried to play down the row, saying it was "nothing new".