Three police forces may not be able to provide a proper service because of the impact of budget cuts, HM Inspectorate of Constabulary has warned.
The policing watchdog has released its latest report on the impact of the government's 20% funding cuts to police in England and Wales by 2015.
It said the Met Police, Devon and Cornwall and Lincolnshire were at risk.
The forces "may not be able to provide a sufficiently efficient, or effective service" in future, it said.
The inspectorate said the 43 police forces in England and Wales would have 5,800 fewer officers in front-line roles as a result of the budget cuts.
The number of civilian staff doing front-line duties would also fall by 2,300 by 2015 compared to five years earlier, it said.
In 2010, the HMIC suggested that a 12% funding cut was achievable but that going further risked damaging front-line policing in England and Wales.
But ministers insist changes to police practices and priorities will increase the number of hours officers spend fighting crime rather than on paperwork and administration.
Official Home Office figures released in January showed that the number of police officers in England and Wales had fallen to its lowest level for a decade.
There were 135,838 police officers in September 2011 - 6,012 (4.2%) fewer than the 141,850 of a year earlier.
'Fearful for safety'Speaking on BBC Radio 5 Live ahead of the report's release, Minister for Policing Nick Herbert insisted it was possible to deal with spending reductions while protecting the front-line services.
"I don't think it's just about the numbers of police officers. I think it's about how visible and available they are, how they're being deployed, how efficiently they're being used and how much bureaucracy they're having to deal with," he said.
"We've never given a guarantee that we can protect the overall numbers. What we've said is that we want to see the front-line service protected and my judgement is that that's what every chief constable is trying to do."
But Police Federation chairman Paul McKeever said Mr Herbert's claim was "extraordinary".
"What we're having fed back to us around the country - from officers who are actually doing the job on the street, when I go and speak at open meetings - is that the front line is being affected very sharply indeed and that officers are being stretched more and more.
"And they are fearful for the safety of the public and also for themselves."
Chief Constable of West Midlands Police Chris Sims told 5 Live that his force would have lost 2,000 jobs by the end of the current four-year cycle but that crime was down and service was improving.
"That's happening because we are able to change at the pace that the cuts are taking place," he said.
"Numbers are important but I also think it's the way policing is organised - our willingness to change - and by doing that I think we are still able to address the concerns of the public in this more challenging time."
