Almost 6,000 officers will be lost from the frontline in three years' time as a result of the Government's budget cuts, figures showed today.
At least 179 police stations will close and one in five will lose their front counters, the report by Her Majesty's Inspectorate of Constabulary (HMIC) found.
Three forces - including Britain's biggest, the Metropolitan Police - may not even be able to provide an efficient or effective service for the public in the near future, the inspectors said.
At least 15,000 officers will be lost as police workforces are cut by 32,400 officers and staff by March 2015, the HMIC report said.
Some 2,700 officers had already been cut from the frontlines by March this year, and this will increase to at least 5,800 (6%) by March 2015, possibly more once figures from the Met and Cheshire are included.
But the proportion of officers on the frontline will increase to between 81% and 95% as the number of non-frontline officers is almost halved, with 7,600 going by 2015, the report said.
In a bid to try to offset the closures of front desks and stations, some 137 police access counters will be set up in libraries and supermarkets.
Sir Denis O'Connor, the Chief Inspector of Constabulary, said forces were "protecting but not preserving the frontline".
While 6% cuts to the frontline could be achieved, a third off the non-frontline with the loss of 20,300 officers and staff would require a "transformation" of policing, he added.
Neither the Met nor Cheshire could provide detailed figures of how the cuts will affect their frontlines by 2015.
The report also found that overall crime levels fell 3% between the end of December 2010 and the end of last year, but there were rises in robbery and "other stealing".
There were also variations between forces, with crime increasing in 11 of the 43 forces in England and Wales.
But so far the public has failed to notice the cuts, an HMIC survey of more than 1,300 people suggested.
More than half (55%) said they had seen no change in the policing of their area in the last year, while a fifth (19%) even said they had seen police more often.
The 43 forces in England and Wales need to close a funding gap of £2.4 billion by 2015 and currently have plans to make cuts of £2.1 billion, the inspectors said.
Some £233 million of the £302 million shortfall comes from the Met which, along with with the Lincolnshire and Devon and Cornwall forces, raised the most serious concerns and needs to have a plan to tackle the shortfall in place by the autumn, Sir Denis said.
"There is a risk that three forces may not be able to provide a sufficiently efficient or effective service for the public in the future," the report said.
The criticism comes after the phone-hacking scandal led to the high-profile resignation of Met Commissioner Sir Paul Stephenson and after Stephen Greenhalgh was appointed as the London Mayor's Deputy Mayor for Policing a month ago.
Sir Denis said: "There has been a pause because of all the changes at the top of the Met, executive and politically, and the Olympics.
"Crime has been bubbling up and down for them and their satisfaction levels are not satisfactory, they're low.
"So they've got limited timescales and a lot to do.
"Are there some concerns? Yes. Should they be able to get on top of it? Yes."
Devon and Cornwall had been making savings and restructuring for years before the comprehensive spending review started, giving it less to cut, while Lincolnshire has a relatively low cost base and a large geographical area to cover, Sir Denis said.
John Tully, chairman of the Metropolitan Police Federation, said rank-and-file officers were being stretched to breaking point.
With rest days being cancelled and restrictions on leave for the Olympics, officers were working flat-out and were "in many cases unrested and exhausted", he said.
"More is being demanded of them every day, and I worry that something will break very soon."
Paul McKeever, chairman of the Police Federation, added: "Whichever way you cut it, the resilience of the police service to be able to react to whatever is thrown at it is being threatened."
Joanne McCartney, chairwoman of the London Assembly Police and Crime Committee, added that the report will "worry everyone concerned with safety and security in our capital city".
She called for London Mayor Boris Johnson and Mr Greenhalgh to recognise "the scale of the problem for policing in the capital as cuts bite".
A spokeswoman said that while finding the savings would be challenging, the force was committed to keeping the capital safe while it does so.
The Association of Chief Police Officers (Acpo) admitted that reducing crime and increasing public confidence in policing will become more difficult over the next few years.
But Policing Minister Nick Herbert said: "This report makes it clear that the frontline of policing is being protected overall and that the service to the public has largely been maintained.
"The proportion of officers on the frontline is increasing, the number of neighbourhood officers has gone up, crime is down, victim satisfaction is improving and the response to emergency calls is being maintained.
"While there are particular challenges in three forces, we know that the vast majority are rising to the challenge of reducing budgets while protecting service to the public."
But shadow home secretary Yvette Cooper said the report showed frontline policing was being badly hit, with "thousands of officers being lost from emergency response and neighbourhood teams".