LONDON (REUTERS) - British Prime Minister Boris Johnson vowed to end the early release of convicted terrorists after an Islamist attacker stabbed two people days after he was set free halfway through his prison term.

Sudesh Amman, who was jailed in 2018 for possession of terrorist documents and disseminating terrorist publications, was shot dead by police on Sunday (Feb 2) after he went on the rampage with a stolen 25cm knife on a busy London street.

Amman had previously praised the Islamic State in Iraq and Syria (ISIS) militant group, shared an online Al-Qaeda magazine and encouraged his girlfriend to behead her parents.

Mr Johnson said the government would announce fundamental changes to the system for dealing with those convicted of terrorism offences, saying he had come "to the end of my patience" with freeing offenders before they had completed their sentences and without any scrutiny.

"I think the idea of automatic early release for people who obviously continue to pose a threat to the public has come to the end of its useful life," he said in a speech.

"We do think it's time to take action to ensure that people - irrespective of the law that we're bringing in - people in the current stream do not qualify automatically for early release."

The government has repeatedly promised tougher rules on terrorism since another former convict killed two people and wounded three more before police shot him dead near London Bridge in November.

Johnson said the instances of deradicalising and rehabilitating Islamists was hard and the instances of success were few.

FAKE BOMB

The attacker, Amman, had recently been released from prison, according to police, having been jailed for promoting violent Islamist material.

He went on the rampage at around 1400 GMT on Streatham High Road and had strapped a fake bomb to his body. He stabbed two people, seriously injuring a man in his 40s, while a third suffered minor injuries caused by shattered glass when police opened fire.

At the time of the attack, Amman was under surveillance by armed police, who shot him dead.

He was 17 and living at home with his mother and younger siblings when he first began committing terrorism offences, according to the authorities. Police became aware of his activities in April 2018 and he was arrested by armed officers in a north London street a month later.

When officers examined his computers and phone, they found that he had downloaded material about making explosives and carrying out terrorist attacks, according to prosecutors.

In November 2018, he pleaded guilty to possession of terrorist documents and disseminating terrorist publications, and the following month, he was sentenced to more than three years in prison.

His mother, Haleema Faraz Khan, told Sky News that he was a"nice, polite boy" who was radicalised online and in prison. She said she had spoken to her son hours before the attack and he had seemed normal when she saw him days before.

"He became more religious inside prison, that's where I think he became radicalised," she said.

Messages showed that he had discussed with his family, friends and girlfriend his extreme views and desire to carry out an attack, often focused on using a knife, prosecutors said.

In December 2017, Amman posted a picture of ISIS leader Abu Bakr al-Baghdadi, who was killed in a US raid in Syria in October, and told his brother in a message that "the Islamic State is here to stay".

He also described Yazidi women as slaves and said the Quran made it permissible to rape them, and in another message he encouraged his girlfriend to behead her parents.

Police said he had shared an online Al-Qaeda magazine with his family and while in a discussion about school with a sibling, he wrote he would "rather blow myself up".

Britain has about 220 people in prison with terrorism convictions.

In 2016, Britain announced plans to isolate radical Islamists in special units in high security jails to limit their ability to influence other inmates amid concerns that prisons were breeding grounds for extremists.

But there have been warnings that the system is failing to address the problem and last month one inmate convicted of terrorism offences attacked prison officers.

Ian Acheson, who carried out a review of the management of Islamist extremists in jail, said the prison service did not have the appetite or aptitude to manage terrorist offenders.

"We may need to accept there are certain people who are so dangerous they must be kept in prison indefinitely," he told BBC radio.

ISIS claimed responsibility for Sunday's attack although police said it was an isolated incident.