MINISTRY of Defence experts and dockyard chiefs yesterday were investigating why a nuclear submarine crashed in its own refitting bay.

Damage estimated at thousands of pounds is thought to have been caused to the Swiftsure class Fleet type submarine HMS Sceptre, which is nearing the end of a two-year refit at Rosyth Dockyard, Fife.

The 3500-tonne vessel shot forward 20 yards in its flooded dock at the weekend, while its main engine and prop shaft were being tested, tearing and bending steam pipes used in the generating system.

According to one dockyard source yesterday, it also dragged a crane from its quayside rails, before settling in its original position, without tumbling into the dock.

HMS Sceptre arrived at the yard, run by Babcock Rosyth Defence Ltd for the MoD, 18 months ago and was nearing completion of her refit.

The ship, capable of continuous patrols at high underwater speed and circumnavigating the globe without surfacing, is armed with Tigerfish torpedoes and the Sub-Harpoon missile, in her hunter-killer role against ships and other submarines.

Her weaponry had already been offloaded before berthing for refit.

No-one was available from Babcock Rosyth Defence Ltd, yesterday but a Ministry of Defence spokesman in London confirmed the incident.

He said it happened during basin testing of the main engines and shaft, using shore steam power.

He added: ''On ordering minimum revs ahead, on remote throttles, the main engines oversped, causing the submarine to break its moorings.

''At no time were personnel, either civilian or military, injured and at no time was nuclear safety compromised.''

An investigation was under way, he said, but routine work on HMS Sceptre would continue.

Many workers learned of the incident only when they arrived for their day shift yesterday.

One said: ''It happened at 10 o'clock at night over the weekend, when there was a skeleton workforce on.

''At maximum shift time, it could have been been more serious, especially when the crane was lifted off its tracks.

''From what we are told, it shot forward and listed, breaking its tow lines. Apparently, there is a huge amount of steam piping burst and bent.''