Joanne McCartney, chairwoman of the London Assembly Police and Crime Committee, said the report will "worry everyone concerned with safety and security in our capital city".
It is disturbing that the Met finds itself one of only three forces which the Chief Inspector judges to be at risk of failing to provide an efficient or effective service to the public in the future.
The conclusions of the HMIC report should impress on Mayor Boris Johnson and his Deputy Stephen Greenhalgh the need for a plan getting to grips with the budgetary challenges facing the Met.
They must show that they recognise, as HMIC does, the scale of the problem for policing in the capital as cuts bite.
John Tully, chairman of the Metropolitan Police Federation, said: "It is difficult to reconcile the scale of the expected savings imposed by the Government cuts against the performance improvement that is anticipated - 'more for less' is just not sustainable in the long term.
"Officers are working flat out at present - in terms of operational policing, 2012 has been the most demanding year in living memory.
"My members' goodwill is being stretched to breaking point, with many rest days being cancelled and an embargo on leave during the Olympic lockdown period.
"They are in many cases unrested and exhausted; more is being demanded of them every day, and I worry that something will break very soon."
He added: "HMIC has rightly highlighted the risk to public safety and the long term ability of forces to effectively combat crime."
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."
Spending on goods and services will also be cut by about £474 million.
At least 179 police stations will also have closed by 2015, and 264 stations will have lost their front counter, HMIC said.
Sir Denis added that police stations, even those which offered no public access, were often seen as a reassuring sign of control in communities.
These will be replaced by 137 police access points in "shared locations", such as supermarkets, it said.
Police workforces will be cut by 32,400 officers and staff by March 2015, including the loss of at least 15,000 officers, the HMIC report said.
Some 2,700 officers were lost from the frontlines by March this year, and this will increase to 5,800 (6%) by March 2015.
The total frontline workforce will also be cut by 8,100 officers and staff, or 6%, while those in non-frontline functions will see the loss of 20,300 officers and staff, a third of the total.
Neither the Met nor Cheshire could provide detailed figures of how the cuts will affect their frontlines.
Sir Denis O'Connor, the Chief Inspector of Constabulary, has said the Met need to have a plan to tackle its £233 million funding gap by the autumn.
Asked about the concerns over the Met, Sir Denis said: "The essence of it is they've got a £233 million gap.
"They've already got plans to take over £500 million out, so this is on top of that.
"It will obviously start after the Olympics."
The Met was named as one of three forces which may not be able to provide an efficient or effective service for the public in the future.
"In our professional judgment (and having considered local context, including police cost to the taxpayer), 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 inspectorate's report said.
These were Lincolnshire and Devon & Cornwall, as well as the Met, the HMIC said.
The figures do not include those for Britain's biggest force, the Metropolitan Police, or for Cheshire, as they have not yet produced their plans.
In the last year, the overall police workforce has been reduced by 17,600 officers and staff, more than half of the total reductions planned by March 2015, the inspectorate added.