Police numbers are at a record high
Spending on police overtime in England and Wales increased by 90% over the past decade, the independent Centre for Crime and Justice Studies says.
The overtime bill was nearly £400m last year, even though there has been a record rise in the number of officers.
The Centre has urged a public debate on the future size and role of the police.
Earlier this year, before the change of government, the Home Office announced that it had a plan to cut overtime by £70m a year.
With Theresa May appointed the new home secretary only on Wednesday, a Home Office spokesman said the department would work with her to establish her priorities and "how best to implement government policy".
The Centre said that, overall, police spending grew in real terms from £9.8bn in 1999 to £14.5bn in 2009, but it was unclear where the money came from or what it was spent on.
By far the major portion of the increased spending - just over 75% - had gone on staffing, with rising numbers across the different staff sectors, the report said.
Andy Tighe, BBC News home affairs correspondent
The report highlights a big increase in the overtime bill at a time when there has been a record growth in police numbers. It might have been expected that with more officers, fewer would need to work extra hours.
There is a wide variation in average overtime payments - in 2006-2007 the average was £209 in Northamptonshire and £4,483 in the Metropolitan Police.
This could reflect contrasting policing environments, with city forces having to deal with terrorism and major demonstrations in recent years. But is this the full picture?
A recent report highlighted officers getting generous overtime allowances for answering the phone on a day off.
The then policing minister said there was a culture that overtime was acceptable, rather than exceptional. The Police Federation says members do not work overtime out of choice, but out of a sense of public duty. And they say younger officers want a work-life balance, so overtime payments are needed to persuade them to work unsociable hours.
In 2009 there were a record 142,151 police officers - an increase of 15,337 since 1998.
The proportionate growth in civilian staff, including police community support offices, has outstripped that in police officers, the report said.
Co-author Roger Grimshaw, research director at the Centre for Crime and Justice Studies, said: "Central government has favoured increased recruitment of police officers, but at the same time civilian staff have grown even more.
"When chief constables query the value of police officer recruitment, how can we be sure about the right balance of spending on employees?
"Will politicians' pledges to protect the 'front line' mean cuts in the back-office functions?"
Chief Constable Peter Fahy, of the Association of Chief Police Officers (Acpo), said: "Overtime can be a flexible way of using officers to deal with the competing demands placed on the service.
"As a 24-hour, seven-day-a-week organisation we have to have the ability to respond to any event or crime at any time.
"Overtime is paid to officers of constable or sergeant rank only and reflects the realities of modern policing. It should always be properly monitored by chief officers and police authorities for cost-effectiveness.
"We are absolutely committed to making serious reductions in spending but we will need some brave policy decisions to ensure that we can do that without impacting on the front line."
The Centre said much of the overall spending increase was paid for from council tax rises.
From 2003-2004 onwards, council tax has met around a fifth of police revenue expenditure.
However, it said that this might not be sustainable in the future.