Defence Secretary Michael Fallon said Britain "wouldn't hesitate" to strike again after the killing of 21-year-old Reyaad Khan in Syria, even as doubts were raised about the legality of the action.
"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," Fallon told BBC Radio 4's Today programme.
"It's extremely dangerous because these are attacks that have been and are being planned against major public events on our streets," he said.
The August 21 strike was revealed by Prime Minister David Cameron in a speech to parliament Monday and was the first such military action by Britain in a country where it is not involved in military operations.
Cameron came under pressure to give further details of attacks allegedly plotted by Khan, who left home to join IS in 2013, as the legality of the move was questioned by opposition MPs and rights campaigners.
Cameron told parliament Monday the strike was "an act of self-defence" since Khan had been planning "barbaric" attacks in Britain against high-profile public events over the summer.
The Daily Telegraph newspaper, quoting unnamed government sources, said Khan was leading a plot to attack VJ Day commemoration services in London attended by Queen Elizabeth II and the prime minister.
Ministers said the decision had been taken on advice from the attorney general, Britain's chief legal advisor.
"The advice here is absolutely clear -- that any country has the right to self defence, to protect itself from armed attack," Fallon said.
- 'Draconian thing to do' -
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However former attorney general Dominic Grieve told the BBC it was possible the government's decision could be "legally reviewed or challenged".
"It is a very draconian thing to do, after all the European Convention on Human Rights and the Human Rights Act give a right to life and the United Kingdom should not interfere with that lightly," he said.
"I very strongly suspect that in view of the fact that this man was a British national with family in this country it will probably lead to a legal challenge in due course."
Two other Islamic State jihadists, including Briton Ruhul Amin, also died but no civilians were harmed, in strikes that were "entirely lawful," the prime minister said.
British tabloids cheered the strike with The Sun's front-page headline reading: "Wham! Bam! Thank You Cam!".
But the main opposition Labour Party has called on the government to publish the legal advice it received and Conservative MP David Davis said there should be a formal check on such decisions.
"The prime Minister and his generals make a decision. Somebody should look and say 'was this the right thing to do, was there enough evidence for this, was this the only way we could sort it out?'.
"Because otherwise it's extra-judicial execution."
Amnesty International UK said Britain had joined the US in conducting "summary executions from the air".
"If we allow this to become the norm, we could have countries all over the world conducting aerial execution of perceived enemies on the basis of secret, unchallengeable evidence," its director Kate Allen said.
British lawmakers voted in 2013 against military action in Syria and Cameron has indicated he would only go ahead with a vote to extend airstrikes from Iraq to Syria if he had cross-party support.
Labour leadership contender and bookkeepers' favourite Jeremy Corbyn, said: "I have questioned the legal basis for the use of drones. Urgent consideration now needs to be given to the appropriate process by which attacks such as this one are sanctioned, on what evidence and on what basis of law."