Britain lifted its national security threat level to critical from severe today, meaning an attack is expected, after a bomb on a London commuter train injured 29 people.

Prime Minister Theresa May said in a televised statement that armed police and members of the military would be seen on the streets in the coming days.

"For this period, military personnel will replace police officers on guard duties at certain protected sites that are not accessible to the public,” she said.

Military personnel will replace police officers on guard duties at certain protected sites that are not accessible to the public

She was speaking after a home-made bomb on a packed rush-hour commuter train in London engulfed a carriage in flames early this morning in Britain's fifth major terrorism incident this year, but apparently failed to fully explode.

Passengers heading into the British capital fled in panic after the blast as the train was about to depart Parsons Green underground station in West London at 8.20am.

Some suffered burns and others were injured in a stampede to escape but health officials said none were thought to be in a serious condition.

Islamic State has claimed responsibility for the blast, accoding to the militant group's Amaq news agency.