Britain's newest nuclear-powered submarine was forced to abandon sea trials and return to port in the Clyde after a system defect was discovered last week.

HMS Ambush – which was handed over to the Royal Navy just last month – was towed back to the Faslane naval base near Helensburgh after coming to an unexpected halt in the middle of Gare Loch on Wednesday.

The £1.6 billion sub, right, was seen to be in difficulties by anti-nuclear campaigners from the Faslane Peace Camp. Crew members were seen standing on top of the vessel, which was venting steam and surrounded by three tugs.

The Ministry of Defence (MoD) has confirmed that the submarine experienced a problem, but declined to give specific details.

Something went wrong that "affected a variety of systems", an MoD spokesman said.

He said: "Following HMS Ambush's maintenance period, undertaken at HM's Naval Base Clyde, an issue with a non-nuclear system was identified. A decision was taken to return it to the base to allow remedial action to take place."

The independent nuclear expert, John Large, said: "The MoD should immediately publish a frank and independent risk assessment.

"We need to know the risks of returning this crippled submarine and its potential radiological impact on nearby Helensburgh and communities further afield. The public deserves to be fully informed if there is any radiological risk and harm deriving from this incident."

Large, who has advised governments on nuclear sub safety, suggested Ambush's propulsion system could have been disabled.

He said: "This would be a completely intolerable event if Ambush was on patrol, during which she is expected to operate at a war footing without need for nearby assistance."

The SNP leader in Westminster, Angus Robertson MP, said he would be demanding details of what happened from the MoD.

"Any incident involving a nuclear submarine is worrying and this is not the first time something alarming like this has occurred in Scottish waters," he said.