The Metropolitan Police currently owes its officers 189,000 rest days, the BBC has found.
Deputy Commissioner Craig Mackey said it followed an "unprecedented year for policing" where the force dealt with multiple terror attacks in 2017.
He said officers had been allowed to carry rest days into 2018 and 2019.
However, the chair of the Met Police Federation, Ken Marsh MBE, has described the amount of rest days being cancelled as "absurd".
Throughout 2017, the Met had to deal with several major incidents including terror attacks on Westminster Bridge, at London Bridge and Finsbury Park as well as a response to the Grenfell Tower fire.
Mr Mackey explained it meant officers had to work rest days, on bank holidays and long hours "in order to get the job done".
"I have been amazed by the continued dedication and commitment that is being shown and we have made efforts to ensure that officers regain their rest days as soon as possible," he said.
However, Mr Marsh believes the cancelled days figure is higher and it is putting a "massive strain" on staff.
"Regardless of what figure it is, the amount of rest days being cancelled is absurd," he said.
"Our officers are not contracted, we serve the Queen and what the public don't understand is that these officers have no choice but to work when days off are cancelled.
"It means officers can't recharge and rest up. It is a massive strain on them."
At one point in 2017, Mr Mackey had to write to retired officers to ask them to consider returning to work with the Met to help with an "unprecedented demand" on its detective capacity.
With a spike in violent crimes across the capital this year and the prospect of US president Donald Trump visiting London, Mr Marsh fears the adverse impact on officers could get worse.
"The best way forward is to recruit more officers," he said.
According to the Met, as of April, it currently has 29,924 officers - a drop from 31,075 officers as of January 2017.
"We will make every effort to ensure that, demand aside, officers can continue to take re-rostered rest days in a timely manner," Mr Mackey said.