Former minister Clare Short has claimed that Prime Minister Gordon Brown was "marginalised" throughout most of the build-up to the war on Iraq.
Ms Short, the former international development secretary who gives evidence to the Iraq inquiry this week, said Mr Brown had feared that former Labour leader Tony Blair would use a quick victory over Saddam Hussein to strengthen his political position at home and remove him from the Treasury.
She said that Mr Brown - who was then chancellor - had not spoken out in Cabinet against the war but did not support it.
Downing Street has said that Mr Brown is keen to take the opportunity offered by an inquiry appearance "to state the case why Britain was right to take the action that it did."
But Ms Short's comments on the BBC's Andrew Marr Show will add to the pressure on Mr Brown when he gives evidence to the inquiry himself in late February or early March.
Mr Blair's former spin doctor Alastair Campbell has told the inquiry that Mr Brown was one of the inner circle of key ministers consulted by Mr Blair in the build-up to the invasion.
But Ms Short insisted he had only come to support the military action on the eve of the invasion after deputy prime minister John Prescott patched up a reconciliation between him and Mr Blair.
"He didn't oppose the war, but he didn't support it. He was pre-occupied with other things," he said.