Mr Otter says the public wants more officers on the streets
Front-line policing in Devon and Cornwall could be cut if funding is reduced, the force has warned.
The chief constable has written to local MPs calling on them to help protect the force's budget.
Stephen Otter said such a "brutal reality" would be unavoidable if predictions of future government spending cutbacks became a reality.
Since taking up office in 2006, Mr Otter has made a priority of putting more police officers on the beat.
In his plea to Devon and Cornwall's MPs, he said if future spending cuts were as severe as some analysts predict, the result would inevitably be the loss of police officers - perhaps as many as 300.
The force has about 3,500 officers. Its grant this year was £186m, but the level of funding has not been guaranteed beyond 2010/11.
The police authority said its forecast showed even a 2% increase in the police grant for 2011/12, with a council tax increase of about 5%, would mean the force having to save a further £5m to keep its present policing activity.
Treasurer John Glasby said that would be a "major challenge".
The police authority said it always looked to plan three years ahead
Plymouth Devonport Labour MP Alison Seabeck told BBC News she had previously discussed the issue with the chief constable and genuinely understood his concerns.
"What's extraordinary about the next spending review is it's running into a general election and we've also had a major recession," she said.
Devon South West Conservative MP Gary Streeter said while he would be "outraged" if the number of front-line officers had to be reduced, he thought the force could save money by adopting more modern administrative methods.
The Home Office said that its police funding formula was designed to make sure that all forces were adequately resourced.
It said: "There has been a 60% increase in funding for police since 1997 and police numbers are at an all time high."
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