The Islamic State militant group, which has said it was behind several attacks on Western cities in recent years, including two attacks in London and one in Manchester this year, claimed responsibility for Friday's bomb attack in London's underground system through its news agency, Amaq.
A home-made bomb on a packed rush-hour commuter train in London engulfed a carriage in flames and injured 29 people on Friday, but apparently failed to fully explode, in Britain's fifth major terrorism incident this year.
Passengers on board a train heading into the capital fled as fire engulfed a carriage at Parsons Green underground station in West London at 8.20am (07:20 GMT), with some suffering burns and other injuries in a stampede to escape, witnesses said.
The London ambulance service initially said it had taken 18 people to hospital, but said none were thought to be in a serious condition.
Prime Minister Theresa May called the incident a "cowardly attack" and said the national threat level had been raised a notch to its highest level, "critical."
Pictures taken at the scene showed a white bucket with a supermarket freezer bag on the floor of one train carriage. The bucket was in flames and there appeared to be wires coming out of the top.
"I was on second carriage from the back. I just heard a kind of whoosh. I looked up and saw the whole carriage engulfed in flames making its way towards me," Ola Fayankinnu, who was on the train, told Reuters.
"A lot of people were trampled on. There were phones, hats, bags all over the place and when I looked back I saw a bag with flames. People were crying, shocked, a few people had been injured, some people had been trampled."
Outside the station, a woman was sitting on a pavement with a bandage around her leg, while armed police patrolled. A Reuters witness saw a woman being carried off on a stretcher with her legs covered in a foil blanket.
In 2005, 52 people were killed when four British Islamists carried out suicide bomb attacks on three London underground trains and a bus and this year Britain has suffered four attacks blamed on terrorists.
May said in the statement on Friday: "My thoughts are with those injured at Parsons Green and the emergency services who, once again, are responding swiftly and bravely to a suspected terrorist incident."
Foreign Secretary Boris Johnson said people should "keep calm" and continue their lives as normal.
Television footage showed passengers being escorted off a carriage while a Reuters witness saw armed police scouring a stationary train and a bomb disposal unit at the scene.
The fire brigade said it had sent six engines and 50 firefighters and London Ambulance said it had sent "multiple resources" including its hazardous area response team to the scene.
Transport for London said there was no service on the western part of the District Line which runs through Parsons Green.
In March this year, a man drove a car into pedestrians on London's Westminster Bridge killing four, before he stabbed a policeman to death outside parliament.
A further 22 people were killed in a suicide bombing at a pop concert in Manchester in May and the following month three Islamist militants drove into pedestrians on London Bridge before stabbing people at nearby restaurants and bars, killing eight.
In June, a van was driven into worshippers near a mosque in north London which left one man dead.
On Thursday, figures showed there had been a record number of terrorism-related arrests in the last year and earlier this week Britain's most senior counter-terrorism officer Mark Rowley said there had been a shift-change in the threat.
In the three years until March this year, police foiled 13 potential attacks but in the next 17 weeks, there were the four attacks while the authorities thwarted six others, Rowley said.