The UK would “not hesitate” to launch more secret drone strikes in Syria to thwart potential terror plots, Defence Secretary Michael Fallon has said.
He said the RAF strike which killed two British Islamic State jihadists was a “perfectly legal act of self defence”.
The men were “terrorists who’d been planning a series of attacks”, he said.
MPs rejected UK military action in Syria two years ago – and ministers are now facing questions over the attack and calls to publish the legal advice.
Cardiff-born Reyaad Khan, 21, was killed in the precision strike in Raqqa on 21 August by a remotely-piloted aircraft, David Cameron told MPs on Monday.
The strike was the first targeted UK drone attack on a British citizen.
Ruhul Amin, 26, was also killed and later identified as a British national from Aberdeen.
Mr Fallon said there was “no other way” of stopping Khan, who the prime minister accused of planning “barbaric” attacks on “high-profile public commemorations” in Britain.
“There are other terrorists involved in other plots that may come to fruition over the next few weeks and months and we wouldn’t hesitate to take similar action again,” Mr Fallon told BBC Radio 4’s Today programme.
Mr Cameron said the attorney general had been consulted and agreed there was a “clear legal basis” for the strike on Khan.
But questions have been raised over the decision, with acting Labour leader Harriet Harman among those urging “independent scrutiny” of the attack.
Former attorney general Dominic Grieve said it was possible the decision could be “legally reviewed or challenged”.
Clive Coleman, BBC legal correspondent
The law on drone strikes is a subject of debate.
Every nation has the right in international law under the United Nations Charter to defend itself. But how broadly should that be interpreted? When does an attack cease to be legitimate self defence?
The US has interpreted the law to justify a campaign of drone strikes in Pakistan and elsewhere. It takes the view that it is in a global war against al-Qaeda, among others, and that its citizens are under imminent threat.
That justifies action anywhere in the world, preferably with the consent of the state on whose soil it takes place, but justifiable even if that consent cannot be obtained.
Some are uneasy about that approach and Britain has never subscribed to it. But the UN has not deemed it unlawful.
Monday’s announcement might appear to some to signal the UK adopting this US stance.
However, the prime minister was careful not to adopt that position. His words kept the British action firmly within the right to self defence enshrined in the UN Charter.
If the intelligence was sufficient to justify the action, it is likely to be widely accepted as legal. Some, however, will continue to dispute its legality.
Labour leadership candidate Jeremy Corbyn said the process by which attacks “are sanctioned, on what evidence and on what basis of law” needed to be urgently looked at.
David Davis, former shadow home secretary, said he believed the strike was justified but warned of “the possibility that this translates or becomes routinised”.
And human rights group Reprieve described the attack as “deeply worrying”.
Mohamed Islam, a family friend of Khan’s, called for an investigation into “the truth”, adding it was a “very sad” situation for his family.
Muslim leaders in Cardiff called for proof that Khan was plotting a terrorist attack on the UK.
Stephen Marvin said his friend Amin had “firmly believed he was fighting for a cause that he believed in and nothing was going to change his mind”.
Frank Gardner, BBC security correspondent
Drone strikes are highly controversial.
Opponents of the policy say they are illegal, immoral and ultimately ineffectual.
They point to evidence that US-operated drone strikes in Pakistan and Yemen have killed hundreds of innocent civilians and generated so much anti-Western hatred in those countries that they end up recruiting more violent jihadists.
Many people also find something repellent about drone operators sitting safely in comfort in a base thousands of miles away from their unsuspecting target.
But proponents argue that drone strikes have been highly effective in disrupting terrorist operations, keeping their leaders constantly on the move and too busy to plan attacks.
Some officials even maintain that the constant targeting of jihadist operators in Pakistan’s Tribal Territories with drone strikes has significantly contributed to preventing a repetition of the 7/7 London bombings.
MPs in 2013 rejected UK military action against President Bashar Assad’s regime in Syria, but last September approved British participation in air strikes against IS targets in Iraq only.
But officials said the UK would “act immediately and explain to Parliament afterwards” if there was “a critical British national interest at stake”.
Mr Fallon said the government was prepared to take planned military action against IS in Syria but a fresh Commons vote would be needed.
“At some point the new Parliament will have to rethink the absurdity of us being able to strike against Isil in Iraq but not being able to strike Isil’s command and control centres in north-east Syria,” he said.