“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.”

Prof Michels said the “flash flame” that witnesses said tore through the carriage, causing dozens of injuries, suggested 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,” he added.

“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.”

Al Qaeda have encouraged followers to use such lights in bomb guides published online.

Terror expert Will Geddes said:  'My suspicion is that Parsons Green was not the intended target.

“Unless a person knows it, it is not going to mean a great deal to you - if it was Paddington or Notting Hill, they are internationally recognised names.”