At prime minister’s questions, the Labour leader attacked the weakness in the Online Safety Bill – itself delayed for three years – saying it allowed one site to “cheerlead for terrorists”.
Mr Johnson appeared to concede the loophole should be closed and agreed to finally bring forward the legislation before Christmas, in the wake of the killing of David Amess.
Amid widespread criticism that the government has dragged its heels, Sir Keir urged the prime minister to make the promised crackdown on online abuse a priority.
“We need to stop online spaces being safe spaces for terrorists.…we need to clean out the cesspit once and for all,” he told the Commons.
Mr Johnson attacked Labour for failing to back a ban on early prisoner release, but Sir Keir – a former director of public prosecutions – hit back, saying: “I prosecuted terrorists and I prosecuted extremists.”