The head of the armed forces at the time of the invasion of Iraq is due to give evidence to the official inquiry into the war.
Admiral Lord Boyce is expected to face questions about the impact of political decision-making on military planning for the conflict.
On Monday, Tony Blair's former foreign policy adviser Sir David Manning told the inquiry that the then Prime Minister first asked what were the options for military intervention in June 2002 - nine months before the invasion.
However, Sir David said that at that stage Mr Blair had been unwilling to make any decisions as he did not want to give any sense that he was not fully committed to resolving the crisis diplomatically through the United Nations.
In September, Mr Blair did agree to allow a military planning team to go to US Central Command in Florida, where US President George Bush had set up an Iraq planning cell, on the understanding that no political decisions had been taken.
At that stage Britain was proposing a limited force "package", largely based on air and naval assets already in the region.
However, Sir David said that over time there was an "acceptance" that Britain would be prepared to deploy a full-scale land force.
Lord Boyce may also face questions about his view of the legality of the invasion, although the inquiry chairman Sir John Chilcot has said that is an issue which they will mainly be considering in the new year.
