Image copyright Met Police Image caption Sudesh Amman was under covert surveillance when he stabbed two people in south London on Sunday

The threat from terrorism is "not diminishing", the head of UK counter-terror policing has warned, as he praised covert officers' response to the Streatham attack.

Officers "calmly ran forward" and prevented more people being injured, Assistant Commissioner Neil Basu said.

Sudesh Amman, 20, was shot dead by police after stabbing two people in south London on Sunday.

Security services knew Amman "posed a significant risk", Mr Basu said.

"But with 3,000 or so subjects of interest currently on our radar and many convicted terrorists soon due to be released from prison, we simply cannot watch all of them, all the time," the Metropolitan Police's assistant commissioner said.

He welcomed government plans to "to keep the most dangerous terrorists locked up for longer".

Amman had been released from prison on 23 January after serving half of his sentence for terror offences.

He was under active surveillance at the time of the attack - which police believe to have be an Islamist-related terrorist incident - and had a hoax device strapped to his body.

Mr Basu praised the "quick reactions" of surveillance officers for preventing more injuries.

"They exemplified the courage and sense of duty that our officers have shown time and time again in their efforts to protect the public from the terrorist threat," he added.

Mr Basu said the attack was the third in as many months, following recent incidents at London Bridge and Whitemoor prison, and said the threat was "not diminishing", despite best efforts.

The UK's terror threat level is currently set at "substantial", meaning an attack is likely.

It was downgraded from "severe", the second highest rating, in November, shortly before the London Bridge attack.

Media playback is unsupported on your device Media caption Video shows moments after police shooting

Earlier Metropolitan Police commissioner Cressida Dick said it was "clearly not possible" to stop every attack and Amman was able to stab people despite being under surveillance because such operations are not "man-to-man marking".

In response to the attack, ministers want to introduce emergency legislation to make terror offenders serve more time in prison - but a former government adviser has warned those plans could lead to a legal battle.

A target of 27 February has been set to get the legislation through Parliament to prevent the early release of any more offenders, according to a Whitehall official.

The official said no terrorist offenders are due to receive automatic release before that date.

The government plans to introduce the legislation in the Commons on Tuesday, with the aim of clearing the House by the time it rises for recess the following Thursday.