Britain's war in Iraq, to be examined in a major report due out Wednesday, instilled a deep sense of distrust towards military intervention that still casts a shadow over foreign policy, analysts say.

The decision to join the US-led invasion in 2003 on the basis of flawed intelligence, the occupation and Iraq's bloody descent into sectarian violence, have been examined in detail by the Chilcot inquiry.

But the experience, including the deaths of 179 British soldiers and tens of thousands of Iraqis, has already left its scars -- on both sides of the Atlantic.

"It has defined Britain's security policy," said Malcolm Chalmers, deputy director general of the RUSI think tank.

"You can trace directly the reluctance of the British government to have boots on the ground in Libya or Syria to the experience in Iraq."

With France, Britain initiated efforts for a NATO-led no-fly zone during the uprising against Libyan leader Moamer Kadhafi in 2011 -- but the mission was limited.

It is also conducting air strikes against the Islamic State group in Iraq and Syria, but only after the House of Commons first voted against the mission in Syria in 2013.

"The debate that took place in the UK parliament was utterly dominated by Iraq," noted Jane Kinninmont, deputy head of the Middle East and North Africa programme at the Chatham House think tank.

"Even before that, back around 2005 when you had the severe ethnic cleansing in Darfur, it had become much harder even by then to argue for humanitarian intervention."

The initial justification for war was that Iraqi leader Saddam Hussein had weapons of mass destruction. But when these were not found, the attention switched to the benefits of having removed a brutal dictator.

"It has had a profound impact on public scepticism about the efficacy of military intervention and in particular, of interventions that are styled as humanitarian," Kinninmont said.

"In the US you have seen some parallels -- it became a major feature of Obama's election campaign that he would get the US out of military engagements in the Middle East."

- 'Policy vacuum' -

Kinninmont noted that Britain had moved towards working with military forces in the region, such as Jordan and the Gulf states, rather than taking action itself.

"The problem is that these forces are still not very strong," she said.

But John Bew, reader in history and foreign policy at King's College London, said Iraq had had a paralysing effect, accusing Britain of having a "non-policy" in Syria for many years.

"We stopped thinking seriously about how to manage down violence, how to stabilise the neighbourhood, how to do things like potentially humanitarian corridors, how to put more diplomatic pressure on the Assad regime," he said.

"There is a vacuum in Western foreign policy," he told AFP.

"We haven't addressed Syria and Libya on their merits so much as having a re-run about debates over Iraq. And at some point that has to stop."

The neo-conservative think tank the Henry Jackson Society also warned against retreating further following the publication of the Chilcot report.

"There are many significant failings and lessons to be learned from the Iraq war, as with any conflict," said its executive director Alan Mendoza.

"But one lesson that must not follow is that intervention is wrong, or that we are somehow responsible for the totality of the turmoil in the Middle East today."

His comments echo those made by former Labour prime minister Tony Blair, the man who took Britain into the war and recently called for Western countries to send in ground troops to defeat the IS group.

"It's not clear to me that even if our policy did not work, subsequent policies have worked better," he told CNN in an interview last October.