• The coastguard tug Anglian Prince, right, helps free the HMS Astute, centre, after it grounded off Skye last month. Picture: PA
An investigation was already being held into the grounding of HMS Astute on a shingle bank off Skye last month after the 1 billion vessel, whose key attribute is stealth, was turned into a tourist attraction.
Now a new inquiry is underway after it was revealed that having survived the incident relatively unscathed, the submarine was damaged in a collision with the tug boat hired to free it.
The Anglian Prince was contracted by the navy to help pull the sub to safety.
But during the operation the towing rope became caught in the tug's propeller and pulled the vessels together, damaging the Astute's starboard foreplane.
A navy spokesman said the sub will be repaired at Faslane and trials will resume in due course.
He added: "The inquiry into the damage sustained by Astute is now complete, although the findings have still to be released to naval officers."
The Anglian Prince, based in Stornoway, is normally under contract to the Maritime and Coastguard Agency, but the Ministry of Defence took over its operation temporarily to help Astute.
It was undamaged despite the tangle with a nuclear submarine and is back in Stornoway, having since helped a cargo boat which got into difficulty off Rum this week.
According to a navy source, the damage to the Astute was "not serious" and will not amount to the many millions of pounds reported.
It will, however, be out of service until the damage is repaired.
"The vessel was able to return to Faslane under its own power. If the damage had been as extensive as people are making out that would not have happened,"the source said.
The 100ft-long Astute, the Royal Navy's fastest and most advanced attack submarine, ran aground near Ashaig Beach on Skye while crew were being transferred from the shore to the vessel during sea trials.
The submarine is understood to have strayed hundreds of yards outside the safe sea lane marked on Admiralty charts, with sources, suggesting the charts it was using were out of date.
The Astute was unable to free itself at first but was refloated with the help of the tug on the first high tide.
The MoD said it was a non-nuclear incident and there was no risk of contamination.
No part of Astute's nuclear propulsion system was damaged nor in danger of being damaged, it said.
The vessel's commander, Andy Coles, 47, could face disciplinary action and even court martial following the incident.
Meanwhile, Western Isles MP Angus MacNeil has called for a debate in parliament over the possibility of the Anglian Prince being removed from service due to government cutbacks.
He highlighted the essential work undertaken by the coastguard tug and warned of the potential environment impact of the loss of the service.
He said: "It is only when a major event happens, such as the grounding of HMS Astute, that the value of the stand-by tug is realised. Huge oil tankers pass east and west of the Hebrides daily, and every few years a nuclear submarine seems to go aground - we can't afford to lose the tug."