Snapshot figures from the Metropolitan Police hint at how a slew of major incidents has put pressure on frontline resources.
While Scotland Yard was not able to produce data relating to the number of rest days owed to its officers in September 2017, the last time the country's terror level was set at "critical", the force said some 184,000 days were due to be re-rostered as of the beginning of April.
Based on the roughly 30,000 full-time equivalent officers in the country's largest police force, it works out at around six rest days owed per member of frontline staff.
The figure is expected to have been higher in September, following the Westminster, London Bridge, Finsbury Park and Parsons Green terror attacks, and the Grenfell Tower fire.
Responding to the Press Association's figures, obtained under Freedom of Information laws, Deputy Commissioner Craig Mackey said: "2017 was an unprecedented year for policing in London.
"We have been clear that in dealing with the terrible events of Westminster Bridge, London Bridge, Finsbury Park and Grenfell fire we have been stretched, and that officers have had to work on rest days, and indeed on bank holidays.
"Officers working on these investigations, and in our communities, have been working long hours, including the cancellation of rest days in order to get the job done and I have been amazed by the continued dedication and commitment that is being shown.
"We have made efforts to ensure that officers regain their rest days as soon as possible and as we draw towards the end of the financial year we have reduced the number outstanding.
"We have allowed officers to carry the outstanding rest days into 2018/19 year and will make every effort to ensure that, demand aside, officers can continue to take re-rostered rest days in a timely manner."
According to the Police Federation, which represents more than 100,000 rank-and-file officers across the country, rest days cancelled more than 15 days prior to an event should be re-allocated within four days of that notification being received.
In the event of a terror attack, police officers routinely find they are kept on duty due to the demands of protecting crime scenes and enforcing cordons, as well as general reassurance.
In that event, they would be paid for the extra shift, the Police Federation said.