Britain's newest nuclear submarine is facing a delay coming to the Clyde due to serious safety concerns, according to a report by the Government's nuclear watchdog.
HMS Ambush, the second of the Royal Navy's new fleet of reactor-driven Astute-class submarines, is nearing completion at Barrow shipyard in Cumbria.
But crucial safety tests on its reactor have been banned by the Office for Nuclear Regulation (ONR) until flaws in the yard's emergency procedures have been fixed.
The submarine cannot go to sea until its reactor has been fully tested in Barrow.
According to the Ministry of Defence, the boat was originally due to begin sea trials from the Faslane naval base on Gareloch "by early 2012".
But progress has been hampered because the Barrow yard's operator, arms giant BAE Systems, has twice failed to account for staff after practice evacuations.
In an emergency exercise on October 19 last year, it missed people due to "a number of anomalies with the personnel electronic accounting system and incorrect personnel actions," said ONR.
The regulatory agency then instructed BAE Systems to rerun the exercise on November 15, but the problem persisted.
The company has been told that it must either sort out the problems or come up with a new system for keeping track of personnel.
"An adequate demonstration of staff accountability is required before permission will be granted by ONR to commence power range testing on HMS Ambush, the next boat due to be taken critical at Barrow," ONR concluded.
HMS Ambush's predecessor, HMS Astute, was dogged by problems during its construction in Barrow, which critics say made it five years late and a billion pounds over budget. In October 2010 it ran aground off the Isle of Skye.
A BAE Systems spokeswoman said: "As part of our drive for continuous improvement in our nuclear emergency arrangements we have taken on board lessons learned from our previous exercise demonstration."
A spokesman for ONR confirmed that it was expecting BAE Systems to again try and prove that its systems for keeping track of staff after evacuations worked.
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