A damning report by safety experts has revealed that staff at Britain’s most important nuclear site did “not have the level of capability required to respond to nuclear emergencies effectively”.

In response to a freedom of information request, the Office for Nuclear Regulation (ONR), an arm of the Health and Safety Executive (HSE), said errors by senior fire officers in a preparedness exercise at Sellafield “could have led to delays in responding to the nuclear emergency and a prolonged release of radioactive material off-site”.

The criticism is revealed at a critical time for the nuclear industry, which is trying to build public confidence after the crisis at the Fukushima nuclear plant while drawing up plans to construct a new generation of atomic power stations in Britain.

It is also an embarrassment to Nuclear Management Partners, the private sector consortium which runs Sellafield and is part-owned by Areva, the French engineering company that has prepared the design for a proposed reactor at Hinkley Point in Somerset.

The initial report from the ONR led to an improvement notice being issued to the Cumbrian site, ordering it to improve its training and wider preparedness to deal with emergencies.

Two HSE fire specialists had watched a safety exercise in December 2011 which tested the Sellafield fire and rescue service’s ability to search for two people after a fictional accident that led to the spillage of radioactive liquid and an aerial release of radioactivity. Although the exercise presented “simple scenarios under ideal conditions”, the service’s “resources were stretched” and “there were insufficient numbers of firefighters to achieve the objectives”, according to the HSE report.