At the moment the whole system works on the basis that criminals organise themselves within police authority boundaries. This is absolutely batty

Home Office Minister Charles Clarke

The system also has the capacity to give bobbies on the beat direct access to the Police National Computer - the central store of police intelligence - so that suspects can be identified - or cleared - instantly.

At the moment police have to radio back to their local station and wait for checks to be made from there.

Each of the special handsets - costing around �8 each - could have an emergency "panic button" enabling an officer in trouble to be located to within inches of his position, even if he is not in a position to radio for help.

Reluctant to share cost

But the Airwave system has already been criticised on technical grounds, because of its cost and also possible dangers to health.

It has been delayed because individual forces are reluctant to find their share of the estimated �2.5bn cost of introducing the system from existing budgets.

West Yorkshire police led what was described as a "rebellion" against introduction until earlier this month Home Secretary Jack Straw pledged an extra �500m to help with set up costs.

At the same time there have been worries that the powerful new digital signals could present a health risk or interfere with sensitive electronic equipment - including some already used by the police and other emergency services.

Last summer's Stewart report into mobile phone safety urged caution with equipment operating at or near 16 megahertz. Airwave operates at 17.6 megahertz.

There is also a possibility Airwave could knock out police radar speed traps, hand-held breathalysers, or make them give false readings.

One of the most serious concerns is the danger of similar interference with equipment in ambulances - since police are frequently at the scene of accidents.

During the trail period test will be made by the Defence Evaluation and Research Agency and, in addition, the National Radiological Protection Board.

Government wants to push ahead

Mr Straw has promised if any potential health threat is detected it will be made public immediately.

But the government is determined to push ahead with the scheme and create a national communications system as soon as possible.

"At the moment the whole system works on the basis that criminals organise themselves within police authority boundaries. I think this is absolutely batty," Home Office Minister Charles Clarke recently told a police industry magazine.

He said: "It's absolutely intolerable if a force won't acknowledge this and it won't be permitted."

Mr Clarke says the government will take special powers to make local forces adopt the system if any remain reluctant.