Midland ambulance staff are preparing to vote over strike action following a pay parity exercise.

Unison, which represents 850 of the 1,500 ambulance staff in Shropshire and the West Midlands, issued the warning after the Birmingham and Black Country Strategic Health Authority intervened in pay talks between the union and ambulance services.

The Agenda for Change talks form part of the biggest shake-up of NHS pay and conditions since 1948 and every job is being graded to ensure consistency throughout the service.

The health authority has told ambulance service bosses not to announce the proposed new wage structure until it has been compared with the same roles in two other ambulance services.

Ray Salmon, Unison's regional organiser, said paramedics and technicians had seen their roles change significantly since a national ambulance service strike in 1989/90.

He said: "They don't want to strike but many of our members feel the work they do is not being recognised."

Technicians, who earn about £20,000, want to be classed as band five grade which would push their pay up to about £24,000.

The salary of paramedics, who get £21,000, could be boosted to between £22,000 and £30,000 at band six.

But Unison fears technicians will see a cut of £1,500 a year if their job is rated at band four under the new system, while paramedics would see no pay increase if they were graded at band five.

Mr Salmon added: "The SHA has intervened because the Department of Health do not want to pay technicians and paramedics at bands five and six - they want to pay at four and five instead.

"We expect to start a secret ballot by next Friday. The only way this action can be avoided is if the SHA withdraw and leave us to carry on with the process."

Last night, a health authority spokeswoman said: "We've not been made aware of any threat of strike action. We've commissioned the process involving West Midlands Ambulance Service and until that process is completed, we can't comment on what that might do."

Steve Parry, spokesman for West Midlands Ambulance Service, said more than 200 different jobs are being graded.

He added: "Our biggest concern is with the grades for paramedics and technicians.

"We've sent these grades to two other NHS trusts for consistency checking and we expect to have the results of that external audit within two weeks. This is really a battle between Unison and the strategic health authority and we're stuck in the middle.

"We're doing everything in our power to avoid a dispute of any sort."