There is "credible" evidence British airstrikes against the Islamic State group have killed civilians in Iraq and Syria, the US military has said.
The Ministry of Defence has continued to deny American reports that some RAF airstrikes against IS have harmed civilians.
The RAF has deployed 4,409 bombs and missiles in the five-year war with IS.
The MoD's approach to identifying civilian casualties has been described as "not fit for purpose".
Airwars, a group which has been monitoring the bombing campaign and its impact on civilians, has obtained details of 11 airstrikes carried out by European nations in which at least 40 civilians were killed.
The UK has admitted the RAF was involved in three of the strikes but still insists no civilians were killed.
One of the incidents involving the RAF was investigated by the BBC in 2018 after a whistleblower inside the US-led anti-IS coalition reported civilian deaths.
The coalition now confirms two civilians were killed.
The MoD claims it has killed 3,964 IS fighters and injured 298.
But so far it has accepted responsibility for just one civilian casualty - and none in the heavily-bombed cities of Raqqa and Mosul, where thousands of civilians are believed to have died.
Airwars has documented reports of nearly 30,000 civilian deaths across Iraq and Syria since 2014.
So far the coalition has accepted responsibility for 1,300, the vast majority killed as a result of US airstrikes.
Chris Woods, the director of Airwars, said: "The fact that the coalition has determined these three RAF strikes to be credible events is very significant."
What are the details of the three RAF strikes?
Mosul, Iraq, 9 January 2017
The MoD said an RAF Tornado hit a truck bomb.
The BBC visited the site in 2018 after being told of likely civilian casualties by a whistleblower inside the coalition.
At the time the MoD said all those killed were "highly likely" IS fighters.
But following the BBC investigation the US now says: "Two civilians were unintentionally killed."
Raqqa, Syria, 13 August 2017
The MoD described it as a strike on enemy fighters using a mortar system in a building.
Airwars documented numerous reports from the ground of as many as 10 civilians killed - including a father and his young daughter, Walid Awad Al Qus and Limar Awad Al Qus.
The MoD said it has seen no evidence of civilian casualties.
But the US military said: "Regrettably 12 civilians were unintentionally killed and six were unintentionally injured as a result of the blast."
Bahrah, Syria, 20 January 2018
The MoD said an RAF Reaper fired a hellfire missile at an IS fighter.
It said the missile had to be diverted in flight when another individual appeared in the target zone.
The unintended target was killed but the MoD said further analysis showed him to be carrying a weapon and he was "very likely" to have been an extremist.
The US report stated: "Regrettably one civilian was unintentionally killed."
Burden of proof 'too high'
The US-led coalition against IS has relied heavily on aerial footage to assess whether civilians have been killed in airstrikes.
That footage is not always conclusive. Aerial reconnaissance cannot, for example, identify when civilians may be hiding in a building or be buried by rubble.
Coalition members said they had made their assessments on a "balance of probabilities".
But Airwars said the MoD has set the burden of proof so high that it is "effectively impossible for the UK military to admit any civilian casualties, even when their closest military ally believes the opposite".
The BBC has spoken to several senior US officers within the coalition.
A number have confirmed the UK, along with other coalition partners, had been alerted or "flagged" incidents in which airstrikes may have led to civilian harm.
On each occasion the RAF concluded there was no such evidence.
But one senior US officer told the BBC the RAF was "looking for certainty" of allegations of civilian harm, often when there was none.
The MoD has not visited a single site in Iraq or Syria to investigate allegations of civilian casualties from the ground.
In a statement the MoD said it examined all evidence available, but has "seen nothing that indicates civilian casualties were caused".
"We always seek to minimise the risk of civilian casualties through our rigorous targeting process, but that risk can never be removed entirely, particularly given the ruthless and inhumane behaviour of the adversary, including the deliberate use of human shields," the statement said.
Justin Bronk, a research fellow specialising in combat airpower at the Royal United Services Institute, said for the MoD "the absence of evidence has been represented as being evidence of absence of civilian casualties".
He believes that early on in the bombing campaign, British rules of engagement - agreed by ministers - led to an unrealistic expectation of zero civilian casualties.
Airwars is now calling for a major overhaul in the way the MoD tracks civilian casualties to bring it in line with Pentagon policies.
Unlike the UK, the US military has a team of specialist military personnel who track and investigate allegations of civilian casualties.
The Pentagon is also required by Congress to publish details of any civilian deaths caused by US military action.
The Chilcot Report into the Iraq war, published in 2016, called on the government to do more to accurately track civilian casualties.
It concluded that after the invasion in 2003 "the government's consideration of the issue of Iraqi civilian casualties was driven by its concern to rebut accusations that coalition forces were responsible for the deaths of large numbers of civilians, and to sustain domestic support for operations in Iraq".
In the fight against IS the RAF have been instrumental in defeating a brutal enemy.
But there is little evidence that it has improved the way it tracks potential civilian harm.
Chris Woods, from Airwars, said: "We're frustrated both by the MoD's poor understanding of the real impact of its airstrikes on civilian populations and and apparent lack of interest to fix this serious problem."