The Ministry of Defence has been reprimanded by Government safety regulators for exposing workers to radiation at the Faslane nuclear submarine base near Helensburgh.

The Office for Nuclear Regulation (ONR) and the MoD's internal Defence Nuclear Safety Regulator (DNSR) took enforcement action in April 2013 over the incident, which happened in August 2012.

A group of workers carried out maintenance in a tank next to a live submarine reactor at Faslane without the proper radiation controls in place. They were irradiated, though the doses were said to be low.

"ONR raised a concern due to the failure to adequately control the work and prevent it taking place when the reactor was operational," said an ONR report on safety at Faslane. "This resulted in a failure to properly designate the area and to properly assess the risks to workers … This resulted in lack of ability to demonstrate that any doses received by the workers were as low as reasonably practicable."

ONR consulted with DNSR and they jointly served the MoD with a formal warning letter, promising a joint inspection of the arrangements for controlling work near submarines at Faslane.

The Sunday Herald has confirmed that this incident is the one rated as most serious in the MoD's 2012-13 health and safety report for the Clyde naval base, released to the newspaper under freedom of information law. The MoD report gives no detail about the incident, except to say that it involved a submarine reactor and was classified as a "category B" nuclear safety event, which can include "an unplanned level of radiation exposure ".

The failure of the MoD report to spell out what had happened was criticised by independent nuclear engineer, John Large. Its report was a "gross misrepresentation" of health and safety performance and possibly a breach of radiation safety reporting rules, he claimed.

"All indications are that this incident involved Faslane personnel working on a radioactive liquor tank connected to a nuclear reactor that was running and critical at the time," Large said. "It is absurd that for this serious radiation incident the radiation exposure of the civilian workforce has simply been brushed over and not properly reported at all."

The MoD stressed that none of the events mentioned in its report caused any harm to the health of any member of staff at Faslane, or to any member of the public.