Budget cuts could leave the Metropolitan Police struggling to provide an effective service while it looks to save another £233 million, according to a report into government funding.
Independent policewatchdog the HMIC has released a report on the impact of government plans to cut police spending in England and Wales by 2015.
The Met, Devon and Cornwall, and Lincolnshire are highlighted as being likely to struggle to provide a "sufficient effective service" after the cuts.
Under the budget plans, the number of frontline officers will be cut by 6 percent - taking 5,800 officers off the beat by 2015. More than two-fifths - 42 percent - of non-front line officers, estimated at about 7,600, will lose their jobs.
The report says: "There are some forces where the data collected and the professional judgment suggest multiple concerns around the position they are in and the current plans to manage the spending review reductions. These are the Metropolitan Police Service, Devon and Cornwall Police, and Lincolnshire Police.
"The Metropolitan Police Service is considered a particular concern because of the size of its outstanding savings requirement; its performance issues; and not least the fact that it accounts for one quarter of policing in England and Wales."
The HMIC, the Inspectorate of Constabulary, said it was concerned that the Met's service would suffer unless structural changes are made. Otherwise essential back-office functions would be handed to frontline officers, meaning more has to be done with fewer people.
The report raises concerns that the Met does not have a plan for the spending review period.
"They have a gap in their financial plans of £233m and are not yet able to say what the shape of their workforce will look like by 2014/15."
The Met released a statement explaining that it had delivered savings of £201m in 2011/12 and would hit a target of £537 by 2015.
It said it was "committed to making reductions by considering how we do things differently and prioritising our service to communities".
It also revealed that its new structure, dubbed "Metchange", would deliver savings as it focused on being as lean as possible.
Policing minister Nick Herbert told the BBC that the report showed that frontline policing was being maintained.
Victim satisfaction 'improving'
He said: "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 fo reducing budgets while protecting the service to the public."
The inspector of constabulary, Zoe Billingham, said: "HMIC is pleased to see that forces have risen to the financial challenge and are generally balancing their books. They are making the difficult decisions that are needed to make savings, while taking steps to protect, although not preserve, frontline services.
"However, the full effects of these choices are in many cases still to be felt. We will continue to monitor the impact of the cuts, both on the service provided to the public and on the British model of policing itself."
The report's findings were declared "disturbing" on BBC news by Police Federation chairman Paul McKeever.
He said: "[Police around the country] are already expressing their concerns about the fall in police numbers - the stretch they're actually experiencing and the fears they have for public safety," he said.
"Police officers are very concerned across the country - not only are they themselves greatly demoralised but they're scared for the public as well."
Under the plans the total police workforce will be expected to have been cut by 32,400 between 2010 and 2015.