Michael Fallon said the Government would “not hesitate” to repeat the action that led to Britons being killed by an RAF drone in the Islamic State stronghold of Raqqah last month.
Prime minister David Cameron revealed jihadist Reyaad Khan, accused by the intelligence services of plotting “barbaric” attacks on “high-profile public commemorations” in the UK, was killed by a missile fired at his vehicle on August 21.
Fallon defended the move and said the Government would use military strikes again if there was no other way to stop the terror cells.
He said: “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 refused to be drawn on the number of terrorists planning attacks against Britain but said it was more than three and revealed plots were also uncovered against Australia and the US.
He said: “It’s extremely dangerous because these are attacks that have been and are being planned against major public events on our streets.
"They are potentially attacks on members of our armed forces and on others, which would be extremely dangerous and would obviously involve the loss of life. Government has a duty, where it has information and the ability to prevent such attacks, government has a duty to deal with it.”
The targeted killing of Khan sparked criticism of a “dangerous precedent” and warnings it will spark a legal challenge.
Fellow British citizen Ruhul Amin was one of two other IS fighters who died in the blast, which came three days before a US drone killed another alleged British plotter, Junaid Hussain.
Events presided over by the Queen, including one in June marking the 70th anniversary of VE Day — were reported to have been among the targets.
The prime minister said the targeting of Khan in Raqqah was justified on the grounds of “self-defence”, as he and Hussain were actively involved in recruiting jihadists and orchestrating a number of plots.
Fallon said the jihadis had been involved directly in plotting with people in Britain but would not confirm if any arrests had been made in the UK.
Meanwhile the father of two men believed to have joined terror group Islamic State said he fears his sons could be next on a British government “hitlist”.
Aseel Muthana, 17, joined older brother Naseer, 21, in Syria last year.
The brothers were close friends with fellow Cardiff man Reyaad Khan, who the UK government confirmed was killed in an RAF drone strike last month.
Their father Ahmed, a retired electrical engineer, said he believes it is only a matter of time before he was told his sons would be killed.
He told The Guardian newspaper: “I am frightened my sons .... could be on a hitlist. I don’t think I will ever see my sons again.”
Muthan’s older son Nasser, who had been a promising medical student, went to the same school as Khan and is thought to have become radicalised at about the same time.
He later turned up on an IS recruitment video alongside Khan in Syria. Then, three months later, younger brother Aseel joined his older sibling, and has since spoken online about how he is “prepared to die” in the name of Islamic extremism.