Explosives experts say the Parsons Green bomb could have killed dozens if it had properly detonated at rush-hour this morning.
Anti-terror police are now examining the bucket bomb which failed to fully go off on a tube train in west London as they try to track down the bomber.
Photos of the explosive show a large builder's tub - believed be packed with explosives - inside a Lidl coolbag, with a string of Christmas tree lights, thought to have been a crude detonator, hanging over the top.
Officers compared the device to those used in the failed 21/7 bombings, in which jihadis attempted to blow up tube trains. In that case, the bombs' detonators went off but the explosives themselves were not ignited.