Stretched Lancashire Police's rest day crisis revealed amid claims of police shortages across England and Wales.
The Police Federation described the situation as being “in crisis”, with the lowest number of police in a generation.
Figures obtained under Freedom of Information laws show there were 37,010 hours and 31 minutes owed to 2,675 frontline Lancashire officers as of September 17 last year - the last time the country’s terror alert was at “critical”.
4,626 rest days were either cancelled, outstanding or waiting to be re-rostered.
Lancashire was among forces that had to deal unprecedented demand on resources, following the terror attack in Manchester.
Giving his reaction, Chief Constable Andy Rhodes today said: “ Working rest days should not be the norm and these figures suggest it has become a routine way of resourcing peaks in demand and large scale enquiries and incidents.
“For example, in Lancashire we have tried to minimise use of rest days working on the fracking operation because otherwise we are taking operational staff away from policing our communities.
"This hasn’t always been possible and it’s a fact that we, alongside many other forces, deployed significant numbers of our staff to the Manchester incident utilising rest day working.
“Rest days are supposed to be used for just that - rest - not as a routine way of filling the gaps.”