The servicing of Britain's Trident nuclear deterrent is set to be privatised as part of a review of the armaments base on the Clyde.

Almost half of the 540 jobs at the Royal Navy Armament Depot Coulport, where Trident warheads are stored and loaded on to nuclear submarines, could be taken over by the private sector after a Ministry of Defence review of the base.

The move was last night attacked by the SNP. The party's defence spokesman, Angus Robertson MP, said it was the latest in a series of "highly questionable"

privatisation initiatives by the Labour government.

"The SNP opposition to the nuclear fleet is absolute, but so long as the Trident system remains on the Clyde I would resist increasing privatisation," said Mr Robertson. "This is one of the most sensitive areas of military technology and there are core military services which could not and should not be privatised."

However, an MoD official said: "What we have effectively is an ageing workforce at Coulport and people are retiring and moving on. Structural changes within the Ministry of Defence mean that recruits are no longer able to pursue a career exclusively devoted to nuclear weaponry, so the Ministry fears a skills shortage."

The core work of the Coulport facility, part of the Clyde Naval base that also includes Faslane, is to unload and store nuclear warheads from Britain's four Vanguard submarines that carry Trident missiles.

The skilled weapons handling and administrative back up undertaken by 180 civilian and 40 service personnel working for the director of weapons at the base will form part of the 18-month review announced by the MoD yesterday.

Commodore Chris Hockley, MoD Defence Equipment and Support Naval Base Commander, said: "We have a proud record of supporting the deterrent and providing highly valued, high-skilled jobs in the local community, but we need to make sure that we are developing our workforce for the future.

He added: "As staff who have had a long and successful career with us are retiring we need to find the best way to recruit new and younger staff. Coulport has a crucial role in the future of national security and we must ensure that it remains properly equipped to play its role in that process with safety continuing to be paramount in all that we do."

The trade unions and local authorities surrounding the base have been consulted on the move. Steve Jary, of the trade union Prospect, said the announcement reflected long-standing union concerns about skills shortages. "We are worried that the preferred solution seems to be privatisation," said Mr Jary. "We have previous experience of privatisation and basically the same level of service is maintained, but at extra cost."

Babcock Marine, which already runs a large part of the Faslane base on behalf of the MoD and also the naval dockyards at Rosyth, would be expected to be a part of the skills solution.

"It may be that the MoD operation at Coulport is coming to the critical stage where it cannot certify or licence its own work," said Mr Jary. "The expertise to carry out and monitor the functions of the base may be running out."

The MoD bases on the Clyde are self-regulating but Scottish CND said the move towards privatisation should open the facilities up to the Nuclear Installation Inspectorate which controls nuclear handling facilities across the UK.