The Great North Air Ambulance (GNAA) has retired one of its helicopters, leaving the rescue service with just one aircraft.
The charity has been operating with two helicopters since the lease expired on an MD902 based at Tees Valley Airport.
Now the "difficult decision" has been made to retire the Bolkov from Otterburn Army Camp in Northumberland, due to ongoing mechanical problems.
Cover for the region will be provided by the GNAA's Cumbria-based aircraft.
The charity, which is moving away from its policy of leasing helicopters, is raising funds to buy three new Dauphin N2 models by the end of 2010.
It said it hoped to buy the first of these, to be based at Durham Tees Valley Airport, in the next few months.
In the meantime, rescue cover for the North East and Cumbria should not be affected as winter is traditionally the quietest time of year, according to the organisation.
Chief executive Grahame Pickering said: "Firstly, due to the advances in clinical care that our crews now undertake, we have outgrown the aircraft - it's physically not big enough for us to treat patients with the new techniques we are constantly introducing.
"Secondly the age of the aircraft has had to be taken into consideration.
"The Bolkov is over 40 years old and sadly has spent more time on the ground with mechanical issues than it has in the air, as parts are very hard to source. This is not cost effective to the charity or public."
The organisation is continuing to appeal for donations to help finance the service, and to raise money for the new helicopters.
More than £250,000 has already been donated following a campaign.
A GNAA spokeswoman added: "We are so grateful to the public for their support and as long as they continue to support us, we will support them."