A British man killed in an unprecedented RAF drone strike in Syria was a prolific recruiter and attack planner for Islamic State, a major new report reveals.

MPs and peers given access to intelligence reports said they were "in no doubt" that Reyaad Khan posed a "very serious threat".

He "orchestrated numerous plots to murder large numbers of UK citizens and those of our allies, as part of a wider terrorist group which considers itself at war with the West," according to the assessment from the Intelligence and Security Committee.

Khan, from Cardiff, was targeted by a remotely piloted aircraft in August 2015 in the Raqqah area of Syria.

The operation that killed the 21-year-old was the first time that the UK had conducted a lethal drone strike against a terrorist target outside of participation in a military campaign.

While Khan was the target of the attack, two other individuals travelling in the same vehicle were also killed, including Ruhul Amin, another UK national.

Then prime minister David Cameron announced that the UK had conducted the drone strike in a statement on the threat posed by IS, also known as Isil or Daesh, in September 2015.

But the new report provides the first detailed account of the intelligence amassed by security services on Khan before he was targeted.

Conservative MP Dominic Grieve, the committee's chairman, said: "In terms of the severity of the threat posed by Reyaad Khan, it appears from the 25 intelligence reports and two formal intelligence assessments that we have seen that Khan was a prolific recruiter and attack-planner."

Alongside another plotter, Junaid Hussain, Khan "encouraged multiple operatives around the world to conduct attacks against the UK and our allies" over the course of nine months, Mr Grieve said.

He added: "They provided practical instructions for the manufacture of bombs, and information on targets."

Hussain, 21, from Birmingham, died in a US air strike three days after Khan was killed.

One MI5 assessment compiled in July 2015 noted that "a body of reliable and corroborated reporting indicates Khan, alongside Hussain, continues to be involved in a concerted and prolific online campaign to recruit, task and encourage operatives in the West to conduct attacks in the name of ISIL."

Another intelligence document describes how Khan was "prominent in attack planning on behalf of Daesh; directly inciting individuals to conduct attacks".

The committee's report disclosed that Khan and Hussain were connected to an unspecified number of the seven major plots thwarted in the UK in 2015.

It said: "It is to the agencies' credit that their investigation of Khan's activities revealed these plots which they were then able to disrupt, thereby avoiding what could have been a very significant loss of life."

The report said it was clear that Khan was orchestrating and inciting a significant number of attacks, some of which could have been launched within a short period of time.

However, the committee said its inquiry has revealed wider policy issues surrounding the strike itself, notably around the ministerial decision-making process and the assessment of collateral damage.

Mr Grieve said: "We are in no doubt that Reyaad Khan posed a very serious threat to the UK.

"There is nevertheless a question as to how the threat is quantified and in this instance whether the actions of Khan and his associates amounted to an 'armed attack' against the UK or Iraq - which is clearly a subjective assessment.

"However, we have been unable to consider how ministers made that assessment since we were denied sight of the key ministerial submission.

"This failure to provide what we consider to be relevant documents is profoundly disappointing. Oversight depends on primary evidence: the Government should open up the ministerial decision-making process to scrutiny on matters of such seriousness."

Earlier this year the Attorney General set out the legal basis for British military strikes against terror targets overseas.

Jeremy Wright stressed in January that it is vital for the UK to have the right to deploy lethal force in self-defence, arguing that the law must keep up with "changing times" after technology made it easier for terrorists to mount attacks.