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.
He claimed the Lib Dems and Tories were focusing on Iraq because they have "nothing serious to say" about the issues facing Britain.
Michael Howard accused the prime minister of deceiving the Cabinet and the Commons over the war.
The Lib Dems said Iraq will "dog" Mr Blair if he wins the election.
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 have become the cloak behind which that had 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...".
Asked how many British personnel had died in Iraq he said: "Well you know the figures for that it's ... seventy to eighty people who died...".
According to the Ministry of Defence the number of service people who have died in Iraq is 86, 49 of whom were killed in action.
Former chief of defence staff, Admiral Sir Michael Boyce, meanwhile told the BBC that he remains 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.
This came in the form of "four or five lines" from the attorney general's office, after the collapse of efforts to secure a second UN resolution on Iraq, he said.
He said that he had been seeking legal assurances since February, and that after the failure to secure a second UN resolution, it was "imperative to have something I could show my troops, for their peace of mind and their families' peace of mind".
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. That's not the way government should be conducted."
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%-39%.