Fleeing commuters left with serious facial burns after a 'fireball flew down the carriage' of the packed train

TWENTY people were injured including a schoolboy in a failed bucket bomb terror attack on the London Underground during this morning's rush hour.

Police confirmed they are treating it as a terrorist incident, amid reports a knifeman was still on the run and police had been called to defuse a second device.

Commuters on their way to work reported a "fireball flying down a carriage" as the bomb, hidden in a bucket in a Lidl bag, detonated inside the carriage.

Prime Minister Theresa May is being keep informed of developments, in what appears to be the fourth major terror attack on British soil in six months, after five innocent people were killed in the Westminster attack in March, 22 were killed in the Ariana Grande bombing in Manchester in May, and eight were killed in the van knife rampage on London Bridge in June.

People fled the station screaming as the blast sent flames flying with a Metro reporter at the scene saying people are "really badly burned" adding that "their hair was coming off".

An onlooker said a "fireball flew down a carriage" with panicked commuters diving out an open door at the South West London station.

A picture believed to have been taken on a District Line train shows a burning plastic bucket with what appear to be wires coming from it.