Al-Qaeda poses a renewed threat to the West as Islamic State (Isis/Daesh) comes under increased pressure, a Whitehall security source told The Times.

"Al-Qaeda has been quietly rebuilding itself," the source said. "They watched Isis become the big kid on the block. Al-Qaeda is biding its time. It will still be there when Isis is done."

It comes with Al-Qaeda affiliates in Syria, Yemen and Afghanistan expanding their strength, and IS losing territory to Western-backed forces on the ground in Iraq.

The comments echo those made by security expert Jeff Weyer, who told IBTimes UK in 2015 that as Al-Qaeda's Syrian affiliate – Jabhat al Nusra, recently renamed Jabhat Fateh al-Sham – lose territory to rival rebels in Syria they will seek to target the West.

"When pressure starts to build on them in Syria, that is when you will see Al-Qaeda perhaps start to commit external attacks, to send the message to leave us alone," he said.

After rebranding itself in August, Fateh al-Sham claimed it had renounced its ties to Al-Qaeda. However, it is thought the move was merely a means of gaining a stronger foothold in Syria, and militants are believed to be actively plotting attacks against the West. A senior group leader was killed in a US air strike in Idlib province days ago.

The group is also expanding its strength in Afghanistan, where the US launched an invasion to oust the group in 2001, and has taken advantage of the war in Yemen to cement its presence there, and is believed to have trained the brothers responsible for the Charlie Hebdo attack in Paris in January 2015.

In recent months Hamza bin Laden, the son of Al-Qaeda's founder Osama bin Laden, has emerged as a new figurehead for the group, and has urged supporters to attack the US and its allies.

Jihadist rivals Isis have lost significant territory in Iraq in recent months, with Iraqi forces and Kurdish militias believed to be weeks away from launching an operation to seize back its Iraqi stronghold Mosul.

Defence Secretary Sir Michael Fallon told the Times: "Al-Qaeda is still alive and kicking in Afghanistan, in Syria, in Yemen and elsewhere."

"It has not been finally defeated and we are very conscious of that . . . It is a very direct threat to the UK. Not just the [defence] ministry but the security agencies are very much aware of that."