Police have said that it was too early to determine the motives of an attacker who stabbed a man at Leytonstone Tube station. The man knifed a train passenger and threatened bystanders at the entrance to the station on 5 December, reportedly screaming "This is for Syria!" in what police described as a terrorist incident.

"It's too early to be absolutely certain about the motives for this attack," said Richard Walton, head of the Counter Terrorism Command at London's Metropolitan Police. "Obviously, the Counter Terrorism Command is investigating it. We've got a number of lines of enquiry and we're pursuing this investigation with some speed to determine what happened, how it happened, and particularly the motive that lies behind the attack.

"A man has been arrested. The police response at the time, from the uniformed officers, was tremendous. They were confronted with a man armed with a knife. They were able to restrain him, using a Taser, and then arrest him, and he's now in custody being interviewed."

Britain is on its second-highest security alert level of "severe", meaning a terrorist attack is considered highly likely, though not imminent, mainly because of the threat posed by Islamic State (Isis) militants in Syria and Iraq.

British war planes joined air strikes for the first time on 3 December, a few hours after Prime Minister David Cameron won parliamentary approval to bomb the IS (Daesh) militant group in Syria after it claimed responsibility for attacks on Paris in November that killed 130 people.

Cameron has said air strikes would not increase the chance of an attack on Britain, since militants already viewed it as a top target, with seven plots foiled over the past year. Britain's worst Islamist militant attack was in July 2005, when 52 people were killed in London by suicide bombers on Underground trains and a bus.