A loss of power at a city dockyard had "potential nuclear implications", a Ministry of Defence (MoD) report said.
The 90-minute power-loss, at Devonport in Plymouth, could have been "catastrophic", a nuclear analyst said.
Caused by a central nuclear switchboard fault at the yard, it was the most potentially dangerous of 50 "events" recorded in 2012.
The Office for Nuclear Regulation (ONR) has issued an Improvement Notice following a number of other incidents.
The ONR said the Improvement Notice was not related to the loss of power in July 2012.
The power loss was regarded as code-B on the sliding scale of "severity" - code A being the most severe.
Power is essential at the dockyard to ensure reactors on the nuclear submarines are kept cool.
John Large, an engineer and independent nuclear analyst, said: "The loss of power was a catastrophic event, which is what happened before the Fukushima meltdown."
The Ministry of Defence (MoD) said it took all safety concerns at the dockyard, which is operated by Babcock Marine, "extremely seriously".
In the "site event" report for HM Naval Base Devonport, the MoD said 50 events in 2012 was a higher number than the two previous years, but "broadly similar" to the number recorded in 2008 and 2009.
The report said it was "reasonable" to expect a rise in the number of events as there had been a corresponding increase in the number of "submarine" days in specially-designed nuclear dockyard berths.
Previously unidentified faults in standby diesel generators meant the secondary power source was unable to re-supply power for more than 90 minutes.
"Through a combination of factors and additional unidentified defects, this defect was revealed, and resulted in an event with potential nuclear implications," the report said.
Devonport resident Ian Avent, who is also the organiser of the Campaign Against Nuclear Storage and Radiation, described the loss of power as "scary".
Plymouth City Council added: "We have previously expressed concerns that we should always be kept fully informed of any incident that is likely to be of wider public concern.
"The council wasn't directly informed about this particular incident at the time or before it was noted in a report published recently."
The council said it would be raising this issue at the "highest level" and seeking assurances that it will be "immediately informed "of any incident of this nature in future.
'No immediate safety impact'
In July, Devonport Royal Dockyard was issued an Improvement Notice following inspections by ONR.
The ONR said this followed a number of incidents at the site in which "operations were not carried out in accordance with the licensee's own operating rules and instructions".
In a statement it said that it was made aware of the incident involving a loss of electrical power in July 2012 and it was "satisfied that Devonport Royal Dockyard's (DRDL) investigation and subsequent corrective actions were appropriate".
It added: "ONR continues to monitor the licensee's progress against its corrective actions, and undertook follow up inspections in April and August 2013.
"The incident referred to is not related to the Improvement Notice issued by ONR on 16 July 2013."
According to the Improvement Notice, the dockyard has until 31 March 2014 to meet its own operating rules and instructions.
The ONR said Devonport Royal Dockyard Limited had taken action to remedy some of the problems identified and it was satisfied there was "no immediate impact on safety", but it believed the Improvement Notice was "necessary".
No-one from the operating firm has been available for comment.
Oliver Colvile, the Conservative MP for Plymouth Sutton and Devonport, said he was very concerned about the situation and intended to speak to Babcock Marine about the actions it was taking.
He said he expected the "highest levels of safety" to be maintained at the dockyard.