Private companies could take responsibility for investigating crimes, patrolling neighbourhoods and even detaining suspects under a radical privatisation plan being put forward by two of the largest police forces in the country.

West Midlands and Surrey have invited bids from G4S and other major security companies on behalf of all forces across England and Wales to take over the delivery of a wide range of services previously carried out by the police.

The contract is the largest on police privatisation so far, with a potential value of £1.5bn over seven years, rising to a possible £3.5bn depending on how many other forces get involved.

This scale dwarfs the recent £200m contract between Lincolnshire police and G4S, under which half the force's civilian staff are to join the private security company, which will also build and run a police station for the first time.