The Metropolitan police, Britain's largest force, has said it would lose 1,000 jobs over the next 12 months

The police have raised the prospect that they could take to the streets in protest at coalition funding cuts, as evidence emerged that as many as 20,000 police service jobs will go over the next four years.

Paul McKeever, the Police Federation chairman, told MPs today that he could not rule out a repeat of the police protest marches three years ago when 20,000 angry officers brought Westminster and Whitehall to a standstill over a failure to backdate their pay.

The potential impact of police funding cuts was spelled out by two chief constables who warned they could lead to police station closures, a fundamental reconfiguration of frontline services and an end to "discretionary activities" currently undertaken by beat patrol and school-based officers.

McKeever told the Commons home affairs committee that the scale of police funding cuts could see the police take to the streets again: "We don't rule anything out at all," he said. "Certainly there are no plans in the short term. There are a number of reviews going on. We're taking an active part in those before we decide whether we're going to go. It might happen."

His warning came as research by the shadow home secretary, Ed Balls, disclosed that just over a third of police forces have already announced that they are to shed 14,500 jobs, including 6,250 police officers, over the next four years. Most of the job losses declared so far are in the large city forces that can be expected to bear the brunt of the cuts. The Met has so far only said that it will lose 1,000 jobs over the next 12 months, with the implication that many more will go during the four-year public spending squeeze.

The Labour research suggests estimates by the Police Federation and others that 20,000 police jobs are likely to go as a result of a 20% cut in Whitehall funding for the police over the next four years are close to the mark.

So far Home Office ministers have refused to speculate how many police jobs might be lost, arguing it depends on how far the funding cuts are mitigated by local council tax increases.

Chris Sims, the West Midlands chief constable, told MPs today that he faces losing 2,250 posts in the next four years, including 1,000 police officers: "I absolutely believe that we can continue to provide the service and protection that our communities need but in taking out, as I will have to, some 2,250 staff posts, that does mean that the way the service is presented will necessarily be different."

He said this year it would be "all about cutting away at things like criminal justice, custody arrangements, call handling and so on", but next year "it is absolutely the case that we will have to look at the configuration of what you will understand as our frontline".

Sims said the funding cuts had hit the West Midlands force three times harder than rural forces because it was much more dependent on Whitehall grants.

Peter Fahy, the Greater Manchester chief constable, who is to axe nearly 3,000 staff over the next four years, including 1,400 officers, said police stations would have to close and "discretionary" activity by patrol officers would be cut. He said the most difficult decisions would come in years three and four, when "we really get into frontline savings".

The Police Federation chairman claimed that police numbers, which recently peaked at 142,000 uniformed officers, could fall to "levels last seen in the bad old days of the 1970s", when the comparison was made on the basis of officers per hundred thousand residents.

He warned that the current ratio of 257 officers per 100,000 could fall to 215 per 100,000.