Serious safety problems have been exposed at a dockyard in Cumbria that makes nuclear submarines after an emergency exercise revealed "confusion" and "extremely poor" procedures, according to a report by government inspectors.

Britain's biggest arms company, BAE Systems, is being forced to rerun the exercise to prove that it can cope with a serious nuclear submarine accident after a string of mistakes that could have put lives at risk had the incident been for real.

A report into the exercise at Barrow dockyard in Cumbria this summer, obtained under the Freedom of Information Act, found that members of a rescue team were prevented from getting to the scene of an imagined reactor accident for 15 minutes because they had to fill in forms about radiation risks.

Exercise Indigo, on 13 July, envisaged a mishap with a nuclear submarine's reactor while it was being turned on for power tests during commissioning, leading to a major release of radioactivity.

According to the official report on the exercise, emergency co-ordinators failed to account for people evacuated from the danger zone, the handling of casualties was "poor" and rescue workers were exposed to large doses of radioactivity.

The company said it was a "learning experience". According to the MoD, there were no plans to switch on any submarine reactors at Barrow before December, when the exercise is expected to be rerun.

"We are working closely with BAE Systems to ensure their plan is robust before it is tested again in December," a spokesman said. "There is no threat to the area."

The Guardian has obtained a formal report sent to BAE Systems by the Health and Safety Executive's nuclear inspectorate, and endorsed by the MoD's internal Defence Nuclear Safety Regulator.

"Weaknesses seriously detracted from the integrated response we would expect to see," concluded the report. The exercise failed to provide "an adequate demonstration" of the company's "operational and tactical response to a reactor accident".

The Barrow shipyard is where BAE Systems Submarine Solutions has been building a new fleet of four £1bn nuclear-powered submarines for the Royal Navy. The first, HMS Astute, was handed over to the Faslane naval base near Glasgow in a ceremony involving the Duchess of Cornwall last month.

The submarine programme, however, has been beset by problems and spiralling costs, and is now under threat from public spending cuts.The MoD has also launched an investigation into an incident on 3 September when a ramp was accidentally dropped from a crane on to HMS Astute while it was being loaded at a berth at Faslane.

Exercise Indigo was designed to test how BAE Systems would manage with a more serious accident. But according to the Nuclear Installations Inspectorate (NII) report, there were numerous problems.

• A "protracted" delay in sending in a team from Cumbria Fire and Rescue Service caused by "briefing and paperwork".

• The time taken to account for people who had to be evacuated from the immediate danger zone around the submarine "was considerably greater that would be expected". This was "a significant failing in the demonstration of the arrangements".

• There was "confusion" for several hours over the location of an injured person and the debriefing of personnel was "extremely poor". One casualty "was advised to go home by one person and to hospital by another".

• There was an "absence of any meaningful advice on the likely radiological consequences of the accident", so "the actions taken to protect the public were not well-informed and grossly over-pessimistic".

John Large, an independent nuclear consultant, said: "This was an exercise that went badly wrong because of lack of planning and emergency preparedness. It revealed a mismatch of resources, gross failures in communication, and cock-ups throughout. There has to be a fundamental rethink before the next submarine nuclear reactor is allowed to go critical at Barrow."

BAE Systems pointed out that nuclear inspectors had praised some aspects of Exercise Indigo. "They also highlighted some areas where improvements were necessary," said a company spokeswoman.

The NII confirmed that BAE Systems "did not provide an adequate demonstration during the exercise" and was being asked to rerun the exercise. But an NII spokesman added: "This does not mean the licensee would not have responded adequately had a real incident occurred, more that some of its arrangements were not as robustly demonstrated as we require."