It is understood an announcement will be made next month after speculation grew this weekend over possible RAF base closures.

The Mail on Sunday reported that the Treasury is pushing plans for a new unified search and rescue (SAR) service to replace RAF, Royal Navy and coastguard helicopters with civilian aircraft and crew.

The newspaper claims the American-owned company Bristow plus Canadian-based CHC Helicopters are the favourites to win a £40 million-a-year contract, possibly running for 25 years, when bids are sought next year.

The Ministry of Defence and the Maritime and Coastguard Agency (MCA) today refused to comment on reports that the changes could see the sale of teams at 12 bases, from Sumburgh in Shetland to Portland in Dorset.

With the MoD looking to replace its ageing Sea King helicopters, a joint steering group involving the MCA has been examining the future make-up of all SAR helicopter services.

A military source today told the Scottish Press Association that the provision of helicopters, crews, training, maintenance and operation of rescue centres were all "up for grabs".

Around 97 out of every 100 emergency operations involving RAF and Navy helicopters involve saving civilians, even though their primary purpose is for military search and rescue.

However, Tim Walker, director of the Glenmore Lodge national mountain training centre, today said RAF crews would lose "essential training" if no longer involved in civilian rescues.

He said: "A first-class rescue network, involving a strong bond between the RAF and voluntary mountain rescue teams, has built up over decades.

"I would be a concerned if that, in any way, this was going to be undermined by unnecessary changes.

"I hope those taking the decision recognise the huge value of the current service and conclude it can't get any better and so leave it alone."

SNP defence spokesman Angus Robertson, whose Moray constituency includes RAF Lossiemouth and RAF Kinloss, also warned against privatisation.

"There is deep concern about the Labour Government's privatisation agenda in the armed forces," he said.

"Recent attempts to privatise defence fire services had to be dropped so I'm unconvinced there is any merit in pursuing that agenda in yet another vital area of provision.

"Many questions remain to be answered about the privatisation of essential lifesaving services."

Tory Shadow Scottish Secretary David Mundell added: "We are opposed to any plans which would see a diminution of existing search and rescue services or closure of bases.

"There is no evidence to suggest that there's any problem with the existing service and we must conclude that this plan, like so many recent MoD decisions, is concerned with cost-cutting.

"If the service is to be effectively privatised, it looks as if it would be US companies that would be running it and I fear they would be driven wholly by a cost culture."

The MoD refused to comment in detail on the reports.

In a statement it said: "We are currently considering the best ways of continuing to provide a top-class search and rescue service and will be making announcements shortly."

An MCA spokeswoman said Bristow had already provided it with helicopters and crew and CHC Scotia was due to take over that contract, providing services from coastguard bases in Sumburgh, Stornoway on the Western Isles, Lee-on-Solent and Portland, from July 2007.

In a statement the agency added: "MCA officers are an integral part of the SAR (Helicopter) project team, which is tasked with future replacement of the UK SAR capability.

"Discussions have been taking place and an announcement is expected next month."