Forty new ambulances worth £4m have been mothballed after a health authority belatedly found that they could not cope with speed bumps.

The unused vehicles, in emergency livery and with blue flashing lights, are being kept in storage in North Yorkshire while modifica tions are made. The Tees, East and North Yorkshire ambulance NHS trust, which won no stars in the most recent government ratings, said the ambulances were highly sophisticated but had an unexpected flaw.

A hydraulic lift fitted at the back made the ambulances too heavy to clear speed bumps and the region's many humpback bridges.

A spokesman for the health service union Unison said that paramedic and driving staff had not been involved in original discussions about the new fleet, which might have prevented the debacle.

The first batch of the consignment is not expected to be refitted for two months, with other vehicles being adapted in Germany before being shipped to the north-east.