At least 15 fighters aligned with the Syrian regime, believed at the time to be from Islamic State, were killed during the airstrike on 17 September, near the city of Deir Azzor.
They were targeted by British, American, Australian and Danish fighter jets and drones.
Some 34 precision-guided missiles and 380 rounds of ammunition were fired at the soldiers, who were not wearing recognisable uniforms or insignia badges.
The investigation revealed the coalition had phoned the Russian military command to notify them their planes would be striking the area.
This was to avoid any mid-air collision between the two sides.
However, the information passed to the Russians was incorrect, with the location given wrong by "several kilometres" - an error put down to a lack of knowledge of the terrain.
The investigating team admitted had the correct location been given, the Russians could have warned the coalition they would be attacking Syrian forces before the airstrike took place.
Shortly after the attack, Syrian President Bashar al Assad claimed it was "definitely intentional".
Brigadier General Richard Coe, the investigating officer, said there were a series of mistakes that led to the strikes.
He admitted the errors ranged from misidentification of targets and communication errors with Russia.
The US military's Central Command said in a statement there were "errors in the development of intelligence, as well as missed opportunities for coalition members on duty to recognise and voice contrary evidence to decision-makers".
"Coalition airstrikes are solely aimed at defeating Daesh.
"We would not and did not intentionally strike known Syrian Regime military units.
"While we cannot completely eliminate the risk of accidental targeting when working in difficult conflict environments, all UK airstrikes are precision-guided, conducted under strict targeting procedures and fully in compliance with the Laws of Armed conflict.
"We work hard to do everything we can to prevent any unnecessary loss of life. This approach contrasts strongly with actions of the Assad Regime and Russia."
Seventy witnesses were interviewed in the internal investigation. The full report will remain classified.