Officers could take to the streets to protest amid fears that policing could return to the "very bad days of the 1970s", the Police Federation has warned.
Paul McKeever, chairman of the federation, which represents rank-and-file officers in England and Wales, said there would be at least 20,000 fewer officers within four years as a result of the Government's spending cuts.
Cuts of this scale could see police take to the streets in protest, he said. "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."
Mr McKeever told the Commons Home Affairs Select Committee: "We're expecting to lose at least 20,000 officers over the next three to four years due to the financial situation."
The number of police officers, per 100,000 of the population, "will fall, if we lose 20,000, to a level below what it was in the very bad days of the 1970s when police forces were stretched", he said.
Asked what he meant by the "very bad days", Mr McKeever said: "It was in chaos. People were leaving in large numbers. Nobody wanted to stay because pay and remuneration were so poor.
"It was that bad that it was compulsory for police officers to work three of their rest days as overtime."