Islamic State and other terrorist groups are planning to target aircraft as they aim to carry out another major attack on the scale of 9/11, a top US security official has said.

Elaine Duke, acting US Homeland Security Secretary, said the groups were using smaller attacks to raise money and “keep their members engaged”.

“The threat is still severe,” she said in London after meeting Home Secretary Amber Rudd.

“The terrorist organisations, be it Isis or al-Qaeda or others, want to have the big explosion like they did on 9/11. They want to take down aircraft, the intelligence is clear on that."

Britain and Europe have seen a string of crude attacks, from the ramming and stabbing attack on London Bridge to the homemade bomb in Manchester.