A source inside the coalition fighting the Islamic State group has told the BBC he believes civilians have been killed as a result of RAF air strikes.
The claim casts into doubt the Ministry of Defence's statement it has seen "no evidence" that its strikes in Iraq have caused civilian casualties.
Other coalition members have alerted the UK to times when civilians may have been harmed, a BBC investigation found.
The MoD says "everything possible" is done to minimise the risk to civilians.
But it has also been discovered that some RAF bombs have malfunctioned and strayed off target.
Estimates for the number of civilians killed in the nine-month battle for Mosul - the last stronghold of the Islamic State group in Iraq - range from more than 1,000 to 10 times that number.
Many were trapped, held hostage by IS who used civilians as human shields.
Some were also the unintended victims of the US-led coalition's bombing campaign, which saw 29,000 munitions dropped or fired in and around the city.
The UK was second to the US in the number of airstrikes it conducted for the coalition, a grouping of 75 countries formed in 2014 to defeat IS.
Only about a dozen members of the coalition conducted airstrikes in and around Mosul, with British warplanes hitting more than 750 targets.
The source, who has not been named to protect his identity, says it was "impossible" to conduct a bombing campaign in highly-populated areas, like Mosul, without killing civilians.
He said he had seen evidence that British airstrikes had caused civilian casualties "on several occasions".
"To suggest they have not - as has been done - is nonsense," he added.
The source described a strike on 9 January last year when an RAF Tornado fired a Brimstone missile on what was described as a "lorry bomb" in eastern Mosul.
It caused a large secondary explosion and the source said he believes two civilians were "almost certainly" killed.
The MoD insisted their "very careful analysis" had concluded that those killed were "highly probably" IS fighters.
It refused a request to view the strike footage it filmed from the air and will not release the co-ordinates of other RAF airstrikes. But it did eventually confirm the location of this one strike.
Many potential eyewitness to the strike were in hiding at the time but some say IS fighters were in the area.
And there were reports of civilians being injured too. One man said the "women and men were screaming in the aftermath. Some of them were wounded."
The incident highlights the limitations of making assessments only from the air.
Both the MoD and the source describe the target as a "lorry bomb" but those on the ground insist it was smaller - the size of an SUV.
Another source in the coalition said it is possible for two different people looking at the same aerial imagery to reach different conclusions. Aerial surveillance cannot see through walls and rubble.
During the BBC's investigation other factors emerged that might cast some doubt on whether British airstrikes have caused civilian deaths including:
- Reports of a number of RAF weapons malfunctions
- Instances of some bombs failing to detonate
- Occasions when bombs have strayed and exploded hundreds of metres off target - hitting buildings that were not even even under surveillance
The MoD insists a "very low percentage" of weapons have malfunctioned but it has refused to give numbers.
But other coalition members have highlighted, or "flagged", several incidents when UK airstrikes may have caused civilian harm.
On each occasion the MoD says it saw no evidence it caused civilian casualties.
Over the past three and a half years the RAF has dropped more than 3,700 bombs and missiles in the campaign against IS in Iraq and Syria.
It has still not claimed responsibility for a single civilian death.
The MoD says it has done "everything possible" to minimise the risk to civilians.
It highlights the professionalism of the RAF crews involved, their "rigorous targeting" procedures and use of "precision weapons". But it has never had anyone on the ground to investigate allegations of civilian harm.
The MoD has been careful in its choice of words. It has also admitted it cannot completely eliminate the risk to civilians.
In saying it has seen "no evidence" of causing civilian casualties it is not ruling out the possibility that it might have happened.
Yet there is also the danger the public might have also been left with the impression that hundreds of British airstrikes in a heavily-populated city, like Mosul, caused zero civilian deaths.
'Coalition destroyed us'
Back in the tightly packed streets of Mosul's old city, which suffered the heaviest bombardment, there seems to be no doubt that airstrikes by the US-led coalition did kill civilians.
Abdel Rahman Ali lost five of his children, when a bomb hit his home, and says 47 civilians were killed as a result of the air strikes in his street alone.
"Nobody destroyed us except the coalition," he said.
Airwars, a group which has been monitoring civilian casualties, says it is likely that between 1,066 and 1,579 civilians died in the fighting in Mosul.
Of the half a dozen coalition members who carried out airstrikes in the city, so far only the US and Australia have accepted responsibility for causing civilian casualties.
At present the coalition has admitted to causing just over 350 civilian deaths in Mosul.
US Central Command is still working its way through a list of other allegations.
Chris Woods of Airwars says: "It's simply fanciful to claim no civilians have been harmed by the RAF.
"We don't live in a world of magic bombs and missiles that only kill bad people."
The reality is that while the UK and coalition airstrikes helped rid the city of the scourge of IS the MoD has carried out no investigations on the ground counting the cost to civilian life.