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.

Streatham attack: Man shot dead by police after stabbings Show all 22 1 /22 Streatham attack: Man shot dead by police after stabbings Streatham attack: Man shot dead by police after stabbings Armed police officers on Streatham High Road. The Metropolitan Police warned people to stay away from the Streatham area boguta37/PA Streatham attack: Man shot dead by police after stabbings The man was shot dead by police after two stabbed in 'Islamist-related' terror attack in London boguta37/PA Streatham attack: Man shot dead by police after stabbings Sudesh Amman, the attacker, has been named as a convicted terror offender who was released from prison days ago Metropolitan Police/PA Streatham attack: Man shot dead by police after stabbings Items left on the pavement at the scene in Streatham High Road Isis supporter Amman was just 18 when he was jailed in December 2018 after police found a stash of bomb-making manuals and other propaganda. Now 20, he had declared his own wish to carry out a terror attack and stockpiled a combat knife, air gun and black flag at his family home in London PA Streatham attack: Man shot dead by police after stabbings He was being monitored by undercover police and being followed as part of a “proactive counterterrorism operation” when he started the rampage in Streatham AP Streatham attack: Man shot dead by police after stabbings Amman was able to stab two people before being shot dead by armed officers outside a busy row of shops Gabriel Video/PA Streatham attack: Man shot dead by police after stabbings A woman reacts as police officers interview people near to the scene Getty Streatham attack: Man shot dead by police after stabbings An armed police officer on Streatham High Road Deputy Assistant Commissioner Lucy D’Orsi said Amman was wearing a fake suicide ves Gully/PA Streatham attack: Man shot dead by police after stabbings Police forensic officers work outside of a Boots store AFP via Getty Streatham attack: Man shot dead by police after stabbings Police officers at the scene Getty Streatham attack: Man shot dead by police after stabbings A police forensic officer works at the scene in Streatham High Road PA Streatham attack: Man shot dead by police after stabbings Items left on the pavement at the scene in Streatham High Road PA Streatham attack: Man shot dead by police after stabbings A police officer on the scene Dave Chawner/SWNS Streatham attack: Man shot dead by police after stabbings Police forensic officers are seen near the site Reuters Streatham attack: Man shot dead by police after stabbings Police officers on the scene outside an Iceland supermarket Andy Bullemor/SWNS Streatham attack: Man shot dead by police after stabbings Police are seen near a site where a man was shot Reuters Streatham attack: Man shot dead by police after stabbings A policeman secures the site of the incident EPA Streatham attack: Man shot dead by police after stabbings A view of the scene Megan Tobeck Streatham attack: Man shot dead by police after stabbings Armed police at the scene in Streatham High Road PA Streatham attack: Man shot dead by police after stabbings A view of the scene Michael Owens Streatham attack: Man shot dead by police after stabbings Police forensic officers at the scene PA Streatham attack: Man shot dead by police after stabbings A Police officer is seen near a site where a man was shot Reuters

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.