Tony Blair is to give evidence to a US task force which is considering changes to military strategy in Iraq.
Mr Blair is expected to brief the task force on UK ideas
The prime minister will speak via video link on Tuesday to members of the Iraq Study Group, led by former US Secretary of State James Baker.
President Bush, whose Republican party was defeated in US mid-term polls, is due to meet the panel the day before.
But the Liberal Democrats have hit out at Mr Blair for talking to the US while resisting a UK parliamentary inquiry.
The panel reportedly thinks that "staying the course" is an untenable long-term strategy and is said to have been looking at two options.
One is the phased withdrawal of US troops, and the other is to increase contact with Syria and Iran to help stop the fighting.
A Downing Street spokeswoman said Mr Blair would ensure Mr Baker and his colleagues were "fully briefed on UK ideas" when he spoke to them.
British officials had been in contact with the bi-partisan panel since it began, and the prime minister's intervention was "a natural extension of that", she said.
But Sir Menzies Campbell, leader of the Liberal Democrats, said: "It is intolerable that the prime minister should be giving evidence to an American inquiry and refusing to do so to a parliamentary one in the UK.
"It is to Parliament that Mr Blair should be giving his explanations and unveiling strategy."
Downing Street however accused Sir Menzies of being out of touch with the public.
"Most people in this country will welcome the fact that the Prime Minister is contributing to the deliberations of the Iraq Study Group. It is a shame that Sir Menzies is not one of them," a spokesman said.
A Conservative-backed attempt to force an immediate inquiry into the government's handling of the Iraq war was seen off by Mr Blair last week, although he denied ruling out an inquiry once the situation in Iraq allows.
The US panel is expected to release its findings before the end of the year.