Police chiefs who decided to prosecute a medical transplant driver for speeding at 104mph defended the move yesterday as a way of ending legal anomalies over the NHS's vital organ-ferrying service.

Senior officers of the Lincolnshire force, who charged the ambulance driver Mike Ferguson, of Bradford, believe the case ultimately could give the service the same immunity as 999 vehicles.

Transplant taxis, although fitted with flashing blue lights and medical equipment, are not protected from speeding prosecutions. An advanced driver with more than 30 years' experience, Mr Ferguson could lose his licence and possibly his job if convicted.

The chief constable of Lincolnshire, Richard Childs, said yesterday that the hearing before Grantham magistrates on June 11 would clarify the reasons for prosecuting. Legal curbs prevented a detailed discussion of the case, he said, but the status of transplant drivers would be an issue.

"Sometimes a decision to prosecute, whilst the right one technically, may not be the right one in the eyes of the wider community until the broader issues are in the public domain," he said. "All I can do is to make it very clear that we consider saving life the most important thing we do."

Mr Ferguson was clocked twice on the A1 in the middle of the night as he ferried a liver from Leeds to an emergency transplant at Addenbrooke's hospital, Cambridge.

He said yesterday: "I would not dream of speeding unless it was an emergency but with some organ transplants, time really is of the essence."