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 that killed two British Islamic State jihadists was a “perfectly legal act of self defence”.
There are “other terrorists” involved in “other plots that may come to fruition” in weeks or months, he said.
The father of two more Britons thought to be fighting in Syria has said he believes they are now on a “hit list”.
A debate over the UK's use of drones has started after MPs were told that Cardiff-born Reyaad Khan, 21, had been killed in a precision strike in Raqqa on 21 August by a remotely piloted aircraft.
Ruhul Amin, 26, from Aberdeen, was also killed.
Another British fighter, Junaid Hussain, was killed by a US drone strike on 24 August and the BBC has seen evidence that suggests he was involved in an active plot against targets in the UK.
Hussain is said to have been involved in encrypted phone conversations with another British man – who cannot be named for legal reasons – who was allegedly at an advanced stage in planning an attack in the UK.
It is understood the plot was disrupted before Hussain was killed.
The RAF strike was the first targeted UK drone attack on a British citizen.
Mr Fallon said there had been “no other way” of stopping Khan, whom Prime Minister David Cameron accused on Monday of planning “barbaric” attacks on “high-profile public commemorations” in Britain.
“We wouldn't hesitate to take similar action again,” Mr Fallon told BBC Radio 4's Today programme.
The defence secretary would not be drawn on whether the UK had a “hit list”, but said: “There are a large number of individuals – not all British – out there in Syria at the moment who are actively involved in planning armed attacks here in Britain.”
MPs rejected military action in Syria two years ago, but Mr Cameron said the attorney general had agreed there was a “clear legal basis” for the strike.
He said it had been approved at a meeting of “the most senior members” of the National Security Council and authorised by Mr Fallon.
The prime minister's official spokesman said the decision had been taken “some months ago”.
The government has justified its decision to launch the strike under Article 51 of the United Nations charter, which says member states have an “inherent right of self-defence” if an armed attack is occurring or is believed to be imminent.
But acting Labour leader Harriet Harman and the SNP's Westminster leader Angus Robertson have called for the Intelligence and Security Committee (ISC) to investigate.
The ISC can summon intelligence chiefs to give evidence, but a new committee has not yet been appointed following the general election.
There have also been calls for the government to reveal details of the intelligence that prompted the strike, including from Labour leadership candidate Andy Burnham, who said it was “unacceptable” for ministers to say that they will not publish any further information.
In other developments:
Ahmed Muthana, the father of Nasser and Aseel Muthana, who are both believed to be fighting with IS in Syria, says he believes the pair could be killed in a similar drone strike because the UK government is “targeting everyone now”
Family friends of Reyaad Khan, along with Muslim leaders in Cardiff, have called for proof of his involvement in a planned attack on the UK
Ibrahim Alwawi, an imam in Aberdeen, said the death of Ruhul Amin was a sad end because he could have been a successful British Muslim rather than a threat to the country.
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.
In 2013, MPs 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 a fresh Commons vote would be needed for pre-planned military action against IS in Syria.