Counter-terrorism officers secretly recorded an alleged Isil-inspired terror cell in Barking last month discussing how to use YouTube to plot a van and knife attack in London, The Daily Telegraph can reveal.

The investigators were monitoring the alleged extremist cell in the east London borough weeks before Saturday night’s attack in the capital, which left seven dead and 48 injured. The Islamic State group has claimed responsibility for the attack.

On Sunday police arrested 12 people, among them seven women – all of them in Barking and other parts of east London, where at least one of Saturday’s killers was believed to have lived. Those arrested were aged from 19 to 60.

Three terrorists shouting “This is for Allah” as they stabbed at their victims were killed by police just eight minutes after the alarm was raised shortly after 10pm on Saturday.