Environmental activists have threatened to bring London's transport network to a halt on Wednesday, after it emerged police do not have enough cells to hold them.

Blockades on four of the capital’s busiest roads were kept in place by demonstrators through Monday night and all of Tuesday, despite almost 300 people being arrested.

Waterloo Bridge, Marble Arch, Oxford Street and Parliament Square were all seized as part of a planned fortnight of disruption by the Extinction Rebellion campaign group.

Fifty-five bus routes have so far been forced to close due to the protests, affecting around 500,000 people, police claimed.

The Mayor of London, himself a passionate campaigner against air pollution, was forced to denounce rumoured plans to bring chaos to the London Underground on Wednesday.

He said: “Targeting public transport in this way would only damage the cause of all of us who want to tackle climate change, as well as risking Londoners’ safety, and I’d implore anyone considering doing so to think again.”

A spokesman for Extinction Rebellion confirmed the group hoped to “escalate” plans to include railways and the tube network.

He added: "I wouldn't expect the entire tube network to be down."