The Liberal Democrats, who opposed the invasion of Iraq in 2003, will try to prevent Jeremy Corbyn from dominating the anti-war side of the argument in the wake of publication of the Chilcot report next month by calling for a two-day Commons debate about the inquiry.

The party’s leader, Tim Farron, said parliament needed maximum scrutiny of the decision to go to war, describing it as the “most damaging decision made by a British prime minister since the second world war”.

Behind the scenes in the Labour party, efforts are under way to reconcile Tony Blair’s determination to defend his decision to go to war with Jeremy Corbyn’s longstanding view that the war was illegal. Some senior shadow cabinet figures are trying to broker a compromise that allows both Blair and Corbyn to state their strongly held positions without starting a political civil war.

In an attempt to ensure that the Lib Dems are not marginalised and that the role of Charles Kennedy, their leader in 2003, is not forgotten, the party is determined to attack the way the war decision was made.

It is expected that the report will be extremely critical of the role of the then head of MI6, Sir Richard Dearlove, and will examine claims that he became too close to Blair and too eager to believe intelligence that Saddam Hussein retained chemical weapons capability. Dearlove has defended himself by arguing that many senior figures in the Saddam regime believed it retained weapons of mass destruction.

Farron said: “This was a foreign policy decision rivalled only by Suez for its folly and incompetence in terms of damaging Britain’s international reputation. We were embroiled in a conflict in Iraq for years that was militarily unwinnable. It was driven by ideologically motivated foreign policy.

“We must have absolute clarity on the decision-making process that led us into Iraq, including the Labour government’s too-close relationship with the Bush administration, and the total mismanagement of the reconstruction effort following the invasion. Given the length of the report and the time period it covers, only a full two-day debate can give us that clarity.”

Senior Labour figures believe that David Cameron will seek to take the heat off the Conservatives in the wake of the EU referendum result by focusing as much as possible on the Iraq war inquiry and a possible vote on the renewal of Trident, two foreign policy issues that Downing Street knows divide Labour.