THE Royal Navy is preparing to scrap HMS Sovereign, one of its 12 nuclear-powered hunter killer submarines, only three years after she completed a #192m refit at Rosyth dockyard in Fife.

The Faslane-based boat, commissioned in 1974, was supposed to remain in service until at least 2005, but has developed serious problems with its reactor which naval sources claim are likely to make further repairs uneconomic.

Sovereign is one of five Swiftsure-class submarines equipped to launch Tomahawk cruise missiles as part of the UK's over-the-horizon strike force.

A sister ship from the First Submarine Squadron on the Clyde, HMS Splendid, fired 17 #750,000-a-shot missiles at targets in Serbia during last year's Nato campaign to free Kosovo.

All 12 of the navy's hunter-killers are now undergoing stringent safety checks after coolant leaks from the reactor on HMS Tireless forced it into an emergency berth in Gibraltar in May. The boat is still there, despite protests by neighbouring Spain over potential radiation risks.

Of the remaining Swiftsure-class vessels, HMS Spartan is in refit at Rosyth and Superb, Sceptre and Splendid are all undergoing safety inspection and modifications and will not be available operationally until well into next year.

All seven of the more modern Trafalgar-class boats are also out of action until the suspected coolant problems are rectified.

The absence of the entire fleet means that protection of the Trident nuclear deterrent and a vital source of intelligence-gathering have both been affected. The hunter-killers normally ride shotgun on the missile boats when they put to sea on patrol.

Outbound protection is being provided by Nimrod anti-submarine aircraft and the United States is believed to be providing additional aid.

Sovereign underwent an overhaul and upgrade at Rosyth from 1992 until 1997. Cracks were then found in her tail shaft during post-refit trials and she returned to Rosyth in June 1998.

The boat has since been used for the annual Perisher nuclear submarine commander's qualifying course in June, 1999 and in May this year. She has also taken part in exercises off the west coast.

In September, the Ministry of Defence admitted that there were ''problems'' with her reactor and she has been undergoing extensive maintenance at Faslane in recent weeks.

A naval source told The Herald: ''Sovereign's problems have reached the stage where the law of diminishing returns has started to kick in. She is the oldest submarine in the fleet. A decision is expected soon on the economic wisdom of keeping her in service. The probability is that she will be decommissioned to cut the drain on the navy's overstretched budget.''

HMS Invincible, the aircraft carrier which played a vital role in the recapture of the Falkland Islands in 1982, is also to be mothballed to save money. The 12 Sea King helicopters assigned to her from the Fleet Air Arm's 814 squadron are being put into storage.

The 21-year-old warship will be placed in ''permanent reserve'' to save her annual #10m running costs. Her sister ships, Ark Royal and Illustrious, are to remain in service until 2012, when they are due to be replaced by two larger carriers.