There will be no Great North Air Ambulance cover for the next few days.
Two of the charity's three helicopters, based at Otterburn and Langwathby, near Penrith, are grounded with mechanical problems.
The lease on the third aircraft, which operates from Durham Tees Valley, runs out at midnight and is not being renewed.
A spokeswoman said everything was being done to restore cover by the weekend or the start of next week at the latest.
The charity was in the process of leasing a new helicopter for its Durham Tees Valley base, and had hoped to have it in place before the existing contract ran out.
It plans to have three larger helicopters in service next year, which it says will provide an improved service.
Grahame Pickering, the charity's chief executive, said it was "bad luck" that the aircraft were temporarily out of action.
The Yorkshire Air Ambulance and the North West Air Ambulance charities also provide cover for Cumbria and North Yorkshire.