Policing a two-week long Extinction Rebellion protest cost Scotland Yard £21 million, the Metropolitan Police Commissioner has revealed.
Cressida Dick said the figure included £3.5 million for overtime, just under £6 million for officers who were drafted in from 38 other forces in England and Wales, and £11.8 million staffing costs.
She added that the bill was set to rise by several million pounds.
The protests, which called on the government to "act now" on climate change, began on October 7 and concluded last week.
Since the start of 2019, Extinction Rebellion protests have cost Scotland Yard a total of £37 million, including earlier protests in April which cost the Met £16 million.
The figure is more than twice the annual budget of its Violent Crime Taskforce, which stands at £15 million.
Dame Cressida said: "We are certainly at the point where I would say to Extinction Rebellion this is placing a horrendous strain on London, and on the Met.
"From the Met's point of view (a) big cost to us and the people who pay for us. Huge drain on our people's resources and energy, causing their families to have to make massive changes in their personal arrangements.
"Frankly, a less good service to the rest of London. Partly because people get tired and partly because we just had to slow down certain types of inquiries, certain types of investigations would just be done more slowly and some things won't ever be done at all."