Nearly 40 police officers from Suffolk and Essex were drafted to London to support the Met during the Extinction Rebellion protests this month.
Under nationally agreed mutual aid protocols, where forces can assist each other in times of need, 25 officers were deployed from Essex, while 12 were sent from Suffolk.
Four officers from Norfolk also supported the Metropolitan Police during the two weeks of climate change demonstrations in the capital.
Metropolitan Police Commissioner Cressida Dick said on Tuesday that the cost of this month's protest action had cost £21million, and is set to rise by several million pounds.
This included £3.5m for overtime, £11.8m in staffing costs, and just under £6m for other officers who were drafted in from 38 forces around the country.
Protest action by Extinction Rebellion (XR) in April this year cost the Met £16m, making a total of £37m - more than twice the annual budget of the force's Violent Crime Taskforce.
MORE: Suffolk Green councillor to face trial over climate change protestsSpeaking on Tuesday, Dame Cressida said policing the protests had put a "horrendous strain" on the force.
She 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."
MORE: Extinction Rebellion protesters target councilMet bosses will apply to the Home Office to cover the cost of the protest action, which saw 1,828 arrests this month.
Rob Cooper, XR spokesman, said: "Perhaps this government should look at how they spend money. According to a report by the European Commission in January our government spends £10.5bn a year subsidising fossil fuels.
"The prime minister recently reminded us that when Margaret Thatcher left office 29 years ago, she was aware of the dangers of global warming but the problem was filed in the 'pending' tray."
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