Frontline police officers in some forces are owed well over a week off in unclaimed rest days, new figures reveal, amid claims of staff shortages across England and Wales.
The union representing rank-and-file officers described the situation as being "in crisis", with the lowest number of police in a generation, concerns over escalating violent crime, and a leaked Home Office report linking the two.
Calum Macleod, chairman of Police Federation in England and Wales, said cancelling rest days - the equivalent of a weekend off during a working week - was having a worrying impact on morale, mental and physical health, and the efficiency of the service.
Snapshot figures obtained by the Press Association under Freedom of Information laws show there were almost a quarter-of-a-million rest days owed to 70,000 police officers in England and Wales as of September 17 last year - the last time the country's terror alert was at "critical" following the Parsons Green terror attack, often resulting in holidays and time off being cancelled.
Some 237,697 rest days were either cancelled, outstanding or waiting to be re-rostered per officer, according to just over 30 forces with data. The true figure is expected to be far higher when including the country's two biggest constabularies - the Met and West Midlands Police, neither of which provided comparable data.
A typical full-time officer would expect to have around nine rest days in a standard month, although some forces said officers could be paid instead of receiving the time back.
Some forces had to deal unprecedented demand on resources, following a string of terror attacks in London and Manchester, and the Grenfell Tower disaster.
The figures come amid a row over police staffing levels after a key Home Office report into tackling violent crime this month failed to acknowledge officer numbers.
Mr Macleod said: "I think this paints a picture of what policing is like in England and Wales at this time - policing is in crisis.
"We do not have the resources at the moment to meet the demands of the public - whether that be in an event, a terrorist incident, or whether that be from a police officer's perspective of actually achieving their rest days.
"It's really important that anybody has rest between their shift patterns because if that isn't happening what you tend to find is people getting fatigued very easily.
"If that isn't happening and rest days are being banked, it's a dangerous situation for the public, it's a dangerous situation for policing and it needs to be addressed.
"The Government needs to take this situation seriously because it's quite clear the model is not working. Officers put their lives on the line for the public day after day.
"They need to listen, they need to listen quickly, because if they don't we are on the brink of disaster."
According to best practice, police forces should re-rota rest days when they are cancelled.
One force, Dorset, provided data which suggested they adhered to this policy completely.
South Wales Police data showed there were 35,857 rest days cancelled, outstanding or to be re-rostered as of September 17, relating to 2,862 officers.
And City of London had 6,601 rest days cancelled for 669 officers - around 10 days owed per officer.
The figures compound Home Office data released last summer that there were 123,142 officers across all ranks in England and Wales at the end of March last year, which the report said was thought to be the lowest number since 1985.
Home Secretary Amber Rudd later told police leaders to focus on cutting crime instead of lobbying the Government for more money.
And the Government's 114-page blueprint to make Britain's streets safe - published on Monday - contained no analysis of any impact from reductions in officer numbers.
Mr Macleod said: "We do not have the resource to engage with the public, all we're doing is fire fighting going from one call to the next call, dealing with one crisis after another.
"That cannot continue. If we cannot engage with the public and gain intelligence from them, our policing model fails in this country."
A Home Office spokesman said: "Policing, by its nature, can be a very challenging and demanding job and it is the responsibility of chief officers - supported by the College of Policing - to ensure that good management systems are in place to support officers in their work.
"Police forces have a statutory duty to manage the working time and welfare of officers and ensure they can take the leave and rest days to which they are entitled."