A rescue squad ambulance caught fire and was destroyed yesterday after it had delivered a patient to Southern Maryland Hospital in Clinton. The ambulance had recently been modified in an effort to prevent fires that have damaged a number of similar vehicles.

No one was injured in the blaze that engulfed the $60,000 Waldorf, Md., Rescue Squad ambulance, which broke out about 3 p.m., Prince George's fire officials said. The fire was discovered by the ambulance driver, who then drove the burning vehicle away from the hospital to a nearby helicopter landing pad, according to William F. Cook, president of the Waldorf Volunteer Fire Department.

The Prince George's Fire Department is investigating the blaze.

The ambulance, a customized vehicle on a Ford Econoline E-350 chassis, is one of a number of ambulances built between 1983 and 1987 that have been voluntarily recalled by Ford because of fires that resulted from fuel escaping from a pressurized fuel system.

Bill Peacock, a Ford spokesman in Dearborn, Mich., said Ford has been told of about 20 cases in which ambulances using the chassis caught fire. As many as 22,000 of all ambulances in the country use the Ford chassis, he said, but added that Ford does not accept responsibility for the fire problem, which he attributed to customizing done by other manufacturers.

Cook said that three weeks ago the ambulance that burned yesterday and a second one were "sent back to the factory in Northern Virginia" where Ford mechanics "removed the gas tanks, then modified the air handling and exhaust unit."