LONDON (Reuters) - Lawmakers investigating a drone strike which killed a British radical in Syria said they were disappointed by a lack of transparency which hindered their efforts to assess whether the strike was proportional.

British national Reyaad Khan and two other Islamic State associates were driving in a vehicle in Raqqa, the de facto capital of Islamic State, when they were killed in the strike on August 21, 2015.

Despite not having a parliamentary mandate to take military action in Syria, then Prime Minister David Cameron told lawmakers at the time that Khan had been targeted and killed in a precision Royal Air Force drone strike.

Cameron described it as an act of self defense, saying they were “seeking to orchestrate specific and barbaric attacks against the west, including directing a number of planned terrorist attacks right here in Britain, such as plots to attack high profile public commemorations”.

It was the first time Britain is known to have conducted a lethal drone strike against one of its own citizens abroad outside a traditional theater of war.

For the strike to be justified under the UN Charter, the strike must be a necessary and proportionate response to an armed attack which is deemed imminent. However, the lawmakers said that they were unable to assess if this was the case, as they were denied a look at a key governmental submission.

“This failure to provide what we consider to be relevant documents is profoundly disappointing,” said Dominic Grieve, the chair of a committee of lawmakers which published its report into UK lethal drone strikes in Syria.

“The Government should be more transparent about these matters and permit proper scrutiny of them.”

The committee said that the threat Khan posed was in directing others to commit attacks against Britain, rather than in launching an attack himself, but said that the “threat he posed was ongoing and there appears to have been no realistic prospect of it diminishing.”

“(The) investigation of Khan’s activities led to the disruption of the attacks he planned, thereby avoiding what could have been very significant loss of life,” Grieve said.