Former foreign secretary, who met with Tony Blair eight days before 2003 invasion, tells Chilcot inquiry he owed the prime minister the 'most robust advice'

Jack Straw, the foreign secretary at the time of the invasion of Iraq, urged Tony Blair just a week before the war to "explore all possible alternatives" to conflict, he told the Chilcot inquiry on Wednesday.

Appearing uncomfortable at times during four hours of questioning, he said he felt he owed the prime minister "the best and most robust advice". He went to see Blair on 12 March 2003 for a meeting for which there is no record available.

However, witnesses have told the inquiry that a decision in principle to join the US-led invasion had been taken long before. And Straw was quick to insist that he personally fully endorsed the decision to invade Iraq.

Straw was giving evidence to the inquiry for the third time in the inquiry's final planned public hearing before the Chilcot team gets down to writing its report, which is unlikely to be published before the summer.

The inquiry made clearer than ever that Blair had gone much further in private letters to President Bush than he admitted in public about the prospect of war to topple Saddam Hussein – an aim of military action that Straw said repeatedly in written and oral evidence would be "palpably illegal".

The letters have not been published but they have been given to the five members of the Chilcot panel. Armed with their contents, Sir Rod Lyne, a former ambassador, asked Straw about a letter Blair wrote to Bush on 3 December 2001. "What I perceive the prime minister was seeking to do was to get onside with President Bush on the issue," Straw replied.

Asked whether Blair was "going down the track of regime change", an objective Straw said would be in breach of international law, the former foreign secretary replied that he did not see papers in which Blair asked for regime change in Iraq. "Ask him," he said referring to Blair.

Straw said he repeatedly warned Blair that a policy of regime change would be "palpably illegal".

Referring to a note he sent Blair in July 2002, Straw said: "I was seeking to persuade the prime minister of my view, not least through my prism of my criticisms of the Americans". He continued: "I don't think [Blair] can be criticised for what he said publicly. I was not present at the private conversations [with Bush]."

Earlier, Straw said he recommended that Blair should read an MI6 paper setting out a "route map for regime change" in Iraq.

Straw was asked why he promoted this document despite his frequently stated position that regime change in Baghdad was not UK policy and could not be a legal justification for attacking Iraq.

He told the inquiry: "As secretary of state I would have read these papers late at night and scribbled on them, 'These are very perceptive, make sure Number 10 see them'. That would have been translated into an official note from my private secretary. That does not mean I've endorsed the policy within those papers."

Lyne asked Straw why he commended a paper that suggested how Saddam Hussein's regime in Iraq could be overthrown.

He said: "I'm very curious you didn't react to the second paper by saying, 'Regime change cannot be an objective for UK foreign policy'."

Straw replied: "The view I have expressed publicly is the same as the views I expressed privately, that regime change was not a good idea for us to pursue as an objective, and in any event it was palpably illegal so it was not an option."

Straw said it was wrong to believe that "everything was pre-planned" and that there was a "sinister design" by Blair and Bush to invade Iraq.

Documents released by the inquiry show that the French government repeatedly complained that Downing Street blamed President Chirac for the failure to get UN backing for an invasion by saying Paris would always veto it.

French foreign minister Dominic de Villepin telephoned Straw insisting that Chirac had "never said" what the British were claiming. "He had not meant that, whatever happened, France would vote 'no'," de Villepin was reported as saying.