The Royal Navy's newest and largest attack submarine HMS Astute has got into difficulty off Skye, the Ministry of Defence (MoD) has confirmed.
An eye-witness said the sub - described as the stealthiest ever built in the UK - appeared to have grounded.
A Ministry of Defence spokeswoman said: "This is not a nuclear incident.
"We are responding to the incident and can confirm that there are no injuries to personnel and the submarine remains watertight."
The spokeswoman added: "There is no indication of any environmental impact."
The Maritime and Coastguard Agency (MCA) said it was alerted to the incident at about 0819 BST.
A spokesman said they were monitoring the situation. A Royal Navy helicopter has also been at the scene.
HMS Astute, built at by BAE Systems in Barrow in Furness, Cumbria, is believed to have been undergoing sea trials as it is not expected to enter service until next year.
Aside from attack capabilities, it is able to sit in waters off the coast undetected, listening to mobile phone conversations or delivering the UK's special forces where needed.
The 39,000 acoustic panels which cover its surface mask its sonar signature, meaning it can sneak up on enemy warships and submarines alike, or lurk unseen and unheard at depth.
The submarine can carry a mix of up to 38 Spearfish heavyweight torpedoes and Tomahawk Land Attack Cruise missiles, able to target enemy submarines, surface ships and land targets, while its sonar system has a range of 3,000 nautical miles.
Speaking to the BBC last month, HMS Astute's commanding officer, Commander Andy Coles, said: "We have a brand new method of controlling the submarine, which is by platform management system, rather than the old conventional way of doing everything of using your hands.
"This is all fly-by-wire technology including only an auto pilot rather than a steering column"
Submarine HMS Trafalgar sustained millions of pounds worth of damage when it ran aground off Skye in 2002.
Two senior commanders were reprimanded after admitting that their negligence caused the incident.
The sea around Skye and the island of Raasay is a training ground for the Royal Navy.
BBC News - Nuclear submarine HMS Astute 'grounded' off Skye
An eye-witness said the sub - described as the stealthiest ever built in the UK - appeared to have grounded.
A Ministry of Defence spokeswoman said: "This is not a nuclear incident.
"We are responding to the incident and can confirm that there are no injuries to personnel and the submarine remains watertight."
The spokeswoman added: "There is no indication of any environmental impact."
The Maritime and Coastguard Agency (MCA) said it was alerted to the incident at about 0819 BST.
A spokesman said they were monitoring the situation. A Royal Navy helicopter has also been at the scene.
HMS Astute, built at by BAE Systems in Barrow in Furness, Cumbria, is believed to have been undergoing sea trials as it is not expected to enter service until next year.
Aside from attack capabilities, it is able to sit in waters off the coast undetected, listening to mobile phone conversations or delivering the UK's special forces where needed.
The 39,000 acoustic panels which cover its surface mask its sonar signature, meaning it can sneak up on enemy warships and submarines alike, or lurk unseen and unheard at depth.
The submarine can carry a mix of up to 38 Spearfish heavyweight torpedoes and Tomahawk Land Attack Cruise missiles, able to target enemy submarines, surface ships and land targets, while its sonar system has a range of 3,000 nautical miles.
Speaking to the BBC last month, HMS Astute's commanding officer, Commander Andy Coles, said: "We have a brand new method of controlling the submarine, which is by platform management system, rather than the old conventional way of doing everything of using your hands.
"This is all fly-by-wire technology including only an auto pilot rather than a steering column"
Submarine HMS Trafalgar sustained millions of pounds worth of damage when it ran aground off Skye in 2002.
Two senior commanders were reprimanded after admitting that their negligence caused the incident.
The sea around Skye and the island of Raasay is a training ground for the Royal Navy.
BBC News - Nuclear submarine HMS Astute 'grounded' off Skye
