It takes two years for a trainee to become fully qualified

The situation is particularly acute in Oxford, where 70% of all officers on the streets have not completed the two-year training period.

The main problem is officer retention, according to the federation, who say that although 470 new officers were recruited last year, about 400 left the Thames Valley force.

The head of the Thames Valley Police Federation told BBC News Online the situation was unfair on both the trainees and the public.

The average length of service of the officers we are losing is between 6 to 15 years - you just can't replace that Inspector Martin Elliot Police Federation

New recruits take two years to become fully qualified officers, and after a year's training they can be working alone.

The ratio of trainees to qualified beat officers across the Thames Valley region is 46%.

Police Federation head, Inspector Martin Elliot said part of the problem was due to the forces' success with recruitment.

"We had about 470 new recruits last year, which is unprecedented in our history - you have to go back to pre-Thames Valley force to see such huge numbers of recruits", he said.

Retention problem

"We have a massive problem with retention though, last year we lost 400 staff.

"The average length of service of the officers we are losing is between 6 to 15 years - you just can't replace that."

The majority of officers leaving the force go to the Metropolitan police, where they can earn between �6,000 and �12,000 more.

The concern is that officers would not have the experience to cope with major incidents, say the federation.

"If there was a serious incident, a bomb blast for example, there is the real risk that we could be losing valuable evidence just because they don't have the skill base", said Inspector Elliot.

"Whilst quite capable of dealing with routine matters there is not the experience to deal with the really serious - it's unfair to expect them to deal with it, and it's also unfair on the public."