The Chief Constable of Thames Valley Police, Peter Neyroud, told BBC News his force is suffering because recruits are thrust into frontline duties too early.

The problem, which is also affecting a number of forces in south-east England, is made worse by experienced officers leaving to join the Metropolitan Police where they receive extra allowances.

Home office minister John Denham told BBC Radio 4's Today programme three south-east forces were in "serious difficulty".

Many of us have been warning for the past year the Home Counties forces - among others - are having extreme difficulties Shadow home secretary Oliver Letwin

"The Met is growing very fast because it provides a counter-terrorism service to the whole country," he added.

Mr Denham said the Home Office would consider giving other forces extra help with housing and encouraging them to increase experienced officers' pay and recruit more people from within their areas.

But Mr Neyroud told the programme the forces needed immediate and long-term help.

In some areas three-quarters of police on the beat were probationers - officers with less than two years experience who have not fully qualified, he said.

The consequence for the public is that you can't get the same level of service from someone who's just joined Martin Elliot, Police Federation

It was putting a "significant strain" on frontline policing, he said, as there were not even enough trained police officers to drive patrol cars.

Shadow home secretary Oliver Letwin said: "Many of us have been warning for the past year the Home Counties forces - among others - are having extreme difficulties.

Officers were frustrated by "colossal bureaucracy and ineffective sentencing of criminals they have spent a long time trying to catch", he added.

Martin Elliot, chairman of the Thames Valley branch of the Police Federation said he was concerned about Thames Valley officers being poached by the Met.

"We've got as many problems in the Thames Valley as they have in the Met, their advantage is that they've got plenty of resources - we're the most under-resourced, under-manned police service in the country."

"The consequence for the public is that you can't get the same level of service from someone who's just joined compared to say a 12 years' service constable or detective."

Free rail travel

Since last April, 138 officers have left Thames Valley for other forces, with more than half going to the Metropolitan Police, which is able to offer up to �6,000 more in allowances and free rail travel.

Surrey Police is also affected - where 200 officers will transfer to other forces this year.

Mr Neyroud said police forces surrounding London needed allowances which kept pace with rising housing costs.

Without extra resources, he said there was a danger that Thames Valley would fail to meet government targets on policing.