Streatham attack: Sadiq Khan expresses anger at Boris Johnson's government over 'preventable' terror attack

London mayor Sadiq Khan has expressed anger at Boris Johnson's government over its failure to prevent a "foreseeable and preventable" terror attack in Streatham.

Police killed 20-year-old Isis supporter Sudesh Amman after he mounted a knife attack on passersby in south London on Sunday.

It emerged that he was being monitored by undercover police after recently being released from prison, where he had served a sentence for terror offences.

In the wake of the attack, the prime minister said he will announce plans later on Monday for "fundamental changes to the system for dealing with those convicted of terrorism offences".

But he came under criticism from Mr Khan, who said the government had failed in its pledge to change anti-terror laws since the incident in London Bridge in November, where two people were killed by convicted terrorist Usman Khan, who had been released from prison on licence.

The London mayor told BBC Radio 4's Today programme: "What makes me angry is we were speaking probably three months ago about the London Bridge attack. In my view, both these attacks were both foreseeable and preventable.

"These were men convicted of terrorist related offences and I have been frustrated for some time about changes to the law in 2012 which took away from judges the power they used to have to give a sentence to protect the public - an indeterminate sentence - and to make sure people weren't released unless we knew they were no longer a danger."

Mr Khan added: "I'm angry because some of the reassurances we were given by the government in November haven't been realised."

Asked if indeterminate sentences should be brought back, he said judges needed the power to give such sentences but also prisons needed better resources to prevent prisoners from being radicalised.

Former head of UK counter-terrorism policing Sir Mark Rowley said there was a case for giving terrorists indeterminate prison sentences but also said there should also be given rehabilitation and de-radicalisation support so they can change their ways.

"If someone is clearly driven by an ideology and they believe that slaughtering other people is a sort of God-given purpose, then I can see a case for that," said Sir Mark.

"As long as we put alongside it the rehabilitation and de-radicalisation programmes to give someone the opportunity to change their ways and be released.

"I don't think there should be a lock-up-and-throw-away-the key - we need to be as equally aggressive about trying to help people turn their lives around as we are determined to protect the public."

Two people were stabbed during the attack in Streatham on Sunday afternoon, including a man in his 40s whose condition is no longer considered life threatening, according to police.

A woman in her 50s has now been discharged from hospital. Another woman in her 20s, who received minor injuries believed to have been caused by glass, continues to receive treatment.

Scotland Yard said armed officers were following the suspect on foot as part of a "proactive counter-terrorism surveillance operation" on Streatham High Road.​

A device found strapped to the body of the suspect was a hoax.

The prime minister paid tribute to the victims and the emergency services, saying an investigation was ongoing into the Streatham attack.

Speaking on Sunday night, Mr Johnson said: "Following the awful events at Fishmonger’s Hall [in London Bridge], we have moved quickly to introduce a package of measures to strengthen every element of our response to terrorism – including longer prison sentences and more money for the police.

“Tomorrow, we will announce further plans for fundamental changes to the system for dealing with those convicted of terrorism offences.”