A “catastrophic” nuclear accident in Plymouth resulting in multiple casualties plunged government agencies into chaos — but luckily for residents it was just a drill.

The October 2013 exercise to test responses to a meltdown aboard a submarine at HM Devonport threw up a string of failings including confusion over what was meant by being “downwind” of radioactive fallout, it was revealed yesterday.

An official report found that dozens of “lessons” needed to be learned from the scenario involving “an unlikely series of catastrophic, highly improbable system and procedural failures.”

The multi-agency drill, dubbed Exercise Short Sermon, saw “severely flawed and wrong” instructions handed out regarding the evacuation of the town and a “lack of understanding” over the direction of a plume of radiation.

A control centre failed to order emergency services near the supposed accident to clear the area of people.

And technical hitches meant that broadband was not “fully available” on the day to those marshalling the response, the report added.