THE improvised bucket bomb left on the Tube at Parsons Green bears the hallmarks of previous failed bombing attempts in the UK, according to experts.

Images of the explosive shared on social media appear to show a bucket placed in a Lidl shopping bag with wires protruding, and reports suggest the device also had a timer.

Academics and chemical experts suggested that, while the device caused what witnesses described as a "fireball", it failed to fully detonate.

The bucket bomb can be seen in this shared video on Twitter.

Dr Lewis Herrington, a lecturer and terrorism expert at Loughborough University, said the use of a timer was "key", as it distinguished it from the suicide attacks on July 7 2005 and at the Manchester Arena earlier this year.

He said: "A timing device is critical - if you look at all the other attacks, the Westminster attack, Khuram Butt, Salman Abedi, they all wanted to die.

"That's not present and that really sends alarm bells.

"This guy really wanted to pack a punch.

"The initiator succeeded hence the reported fireball but clearly the explosive element failed."

Dr Herrington said the attack instead bore echoes of the case of Damon Smith, 20, who was jailed for 15 years in May after leaving a home-made bomb in a rucksack containing ball bearing shrapnel on a Jubilee Line train.

Smith, who had autism and was not motivated by terrorism, created the bomb after finding an al Qaida guide online.

Herrington said such guides could be found within "10 seconds" through search engines.

Chemistry expert Professor Hans Michels said there were similarities with the failed attempts to bomb the Tube on July 21 2005.

The Professor of Safety Engineering, from the Department of Chemical Engineering at Imperial College London, said: "In appearance and arrangement the remnants of the device seem highly similar to those of the hydrogen peroxide-based devices of 2005.

The size of the device and its containment in a plastic bucket is also the same.

"The flash flame reported suggests that the 'explosion' was only partly successful. In particular much of the bucket still seems to be intact and there appear to be no victims with lethal impact wounds.

"I must speculate that either the mixture was not of the right composition or that the ignition system was inadequate or not properly placed. The fact that much of the bucket is intact supports the latter possibility."

The trial of the July 21 bombers heard that the second set of bombs failed because the bombers had got the chemical recipe wrong.