British Prime Minister Boris Johnson said on Friday it was important to enforce terrorism-related sentences, as The Times newspaper reported that a London knife attacker had been convicted of an "Islamist terrorism-related offense" and released from prison about a year ago.

"It is a mistake to allow serious and violent criminals to come out of prison early and it is very important that we get out of that habit and that we enforce the appropriate sentences for dangerous criminals, especially for terrorists," Johnson said.

Britons head to the polls on Dec. 12 to elect a new government.