British Prime Minister Boris Johnson says he will strengthen prison sentences, vowing to boost security after an attack in London by a man jailed for terrorism who had been released early.

Key points: The Conservative Party have promised to change existing laws so those convicted of terrorism offences serve every day of their sentence

The Conservative Party have promised to change existing laws so those convicted of terrorism offences serve every day of their sentence Opposition Leader Jeremy Corbyn says that early releases of prisoners, even those convicted of terror offences, depends on their circumstances and behaviour

Opposition Leader Jeremy Corbyn says that early releases of prisoners, even those convicted of terror offences, depends on their circumstances and behaviour The debate on national security has begun ahead of the December 12 election and after Usman Khan killed two people in a terror attack

Less than two weeks before Britain heads to the polls, law and order has taken top billing since Usman Khan, wearing a fake suicide vest and wielding knives, killed two people on Friday before being shot dead by police.

Mr Johnson said if he won the December 12 election, he would invest more in the prison system and toughen sentences.

"I think it is repulsive that individuals as dangerous as this man should be allowed out after serving only eight years and that's why we are going to change the law," Mr Johnson told BBC's Andrew Marr Show.

He portrayed his rival for prime minister, Jeremy Corbyn, as weak on crime, blaming the Labour Party for a law passed more than a decade ago that provided for some prisoners to be released early automatically.

Mr Corbyn, a veteran peace campaigner, said convicted terrorists should "not necessarily" serve their full prison terms.

"It depends on the circumstances, it depends on the sentence, but crucially it depends on what they've done in the prison," Mr Corbyn told Sky News.

Mr Johnson's Conservatives quickly tried to capitalise on Mr Corbyn's comments with a tweet, in a clear indication that the extremist attack is now fair game for political brinksmanship.

"We will change the law so terrorists serve every single day of their sentence. Who do you trust to keep you safe?" the tweet read.

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Mr Corbyn said Conservative cuts to community policing and probation, mental health, youth and social services could "lead to missed chances to intervene in the lives of people who go on to commit inexcusable acts".

"You can't keep people safe on the cheap," Mr Corbyn added.

The accuracy of Mr Johnson's claim was challenged by Ed Davey, deputy leader of the Liberal Democrats, who told Sky News that the Prime Minister was misleading the public about the current law regarding the early release of prisoners.

"Either he's incompetent and doesn't know the law, or he's deliberately misleading people when we've got a tragedy on our hands, and I'm afraid, either way, it does not look good for the Prime Minister," Mr Davey said.

He said Mr Johnson had a track record of misleading the public on this and other matters, including Brexit.

"I'm really alarmed that we are on the brink of having a prime minister who is the most untruthful prime minister of all time," Mr Davey added.

Khan's attack, which took place on London Bridge, stirred memories of the last election year, 2017, when three terrorists killed eight people and injured at least 48 in the same part of London.

Sorry, this video has expired Members of the public praised for heroism

The Ministry of Justice has begun an urgent review of cases like Khan's that might pose a threat, including a review of the conditions governing the movements of every convicted terrorist who has been released from prison.

Police have said that Khan appears to have been in compliance with the conditions governing his release, which were not made public, but nonetheless was able to carry out a deadly assault that did not rely on sophisticated weaponry or detonation of an explosive device.

The two people killed were Jack Merritt, 25, and Saskia Jones, 23. Both were graduates involved in a prisoner rehabilitation program that was organising a conference that Khan was attending, where he chose to start his attack.

Two people remained in hospital in a stable condition on Sunday, while a third left hospital.

AP/Reuters