Tony Blair has played down a leaked memo indicating he was looking at ways to justify war with Iraq in July 2002 - eight months before the conflict.
The prime minister has again been facing questions over Iraq
He claimed the Lib Dems and Tories were focusing on Iraq as they had "nothing serious to say" about other issues.
Michael Howard accused the prime minister of deceiving the Cabinet and the Commons over the war.
The Lib Dems said Iraq would dog Mr Blair if he won the election, and he would be a "lame duck" prime minister.
The Sunday Times has published what it says is a leaked memo dated 23 July 2002 by Matthew Rycroft, a former Downing Street foreign policy aide.
In the memo, Foreign Secretary Jack Straw is quoted as saying US President George Bush had "made up his mind to take military action even if the timing was not yet decided. But the case was thin".
It adds: "Saddam was not threatening his neighbours, and his WMD capability was less than that of Libya, North Korea or Iran.
"We should work up a plan for an ultimatum to Saddam to allow back in the UN weapons inspectors. This would help with the legal justification for the use of force."
The memo followed a meeting, attended by Mr Blair, Mr Straw, Defence Secretary Geoff Hoon and the attorney general.
Mr Blair told BBC One's Breakfast with Frost the decision had not been taken at that point to attack Saddam Hussein.
"You have got to discuss everything as you go along, but the point is that after that meeting we decided to go back to the UN and give him a last chance.
"If the UN resolution had been adhered to by Saddam that would have been the end of it, despite the fact it was the most appalling regime."
Lib Dem foreign affairs spokesman Sir Menzies Campbell said: "Regime change was always the objective - weapons of mass destruction became the cloak behind which that was been hidden."
In a separate interview with Independent Radio News, Mr Blair repeated his apology for the erroneous intelligence presented by the government in the run-up to the war.
He added: "We do say sorry for all those people who have died but I cannot apologise for taking the country to war...".
Former chief of defence staff, the then Admiral Sir Michael Boyce, meanwhile told the BBC he remained satisfied that he had unequivocal "top legal cover" for his forces when they went to war in 2003.
He was responding to an article in The Observer which suggested he had concerns over whether he could face prosecution over the Iraq war.
Lord Boyce told the BBC he sought, and received, "in black and white terms", the legal assurance he needed prior to the war.
Michael Howard told the Sunday Telegraph Mr Blair had though it legitimate to "dissemble" on Iraq to Lord Boyce.
Defending his accusation that Mr Blair had lied about the war, he told the paper: "What's worse? Accusing someone of lying? Or taking the country to war on a lie? This is perfectly justified.
"Whatever the consequences, you can't maintain a position which says that it's legitimate to trick the Cabinet, to deceive the House of Commons and to dissemble to the Chief of the Defence Staff."
Labour later released some of the campaign literature which they said the Conservatives were planning to use in the final days of the campaign.
Mr Blair's former communications director Alastair Campbell said the Conservatives aimed to convince unhappy Labour supporters the election was a foregone conclusion, so they could safely cast a protest vote for the Lib Dems or not vote at all.
The Conservatives insist they will be offering a positive message, with Mr Howard focusing on the actions he would take as prime minister. They say the focus should not be on tactics, but policy.
All three party leaders were out on the campaign trail on Sunday. Tony Blair was introduced at a rally in Enfield by DJ Chris Evans, who gave Labour a ringing endorsement. Michael Howard visited Newquay in Cornwall, and Charles Kennedy addressed a rally in Newbury, Berkshire.
Latest polls suggest Labour still has a strong lead while the Liberal Democrats are in their best position since the party was formed.
Three polls published on Sunday give Labour a vote share of between 36% and 39%.