Two men have been accidentally sealed inside a submarine ballast tank in Plymouth, it has been reported.

The BBC reports that the electricians were sealed inside by mistake and only saved when one of the men managed to get faint mobile phone signal. They had to climb the inside of the tank to reach a height where their phones received one bar of signal and they were able to message for help.

The event happened in December of last year at a dry dock in the Devonport naval base whilst the men were carrying out safety checks on the vessel. Once they had completed their work and attempted to leave, they were horrified to find that the entrance had been sealed by other workers in preparation for an air tightness test.

Speaking to the BBC, Unite union described the incident as “an extremely unpleasant situation.”

“The incident was actually caused by poor management and poor communication.”

A representative from Babcock International who operate the Royal Navy base, said in a report on the incident that an internal investigation had been carried out and that changes had been made to the company’s work practices. They told the BBC that they are “continually focused on delivering and maintaining the highest standards of safety procedures and practices.”

Devonport is the largest naval base in Western Europe, stretching across 650 acres. The Royal Navy have been based at the site since 1691. Nuclear submarines which are no longer required are sent there for fuel to be disposed of.

In 2013 the Office for Nuclear Regulation said it served an improvement notice on Devonport for three alleged breaches of health and safety legislation, and of Section 24 of the Nuclear Installations Act regarding “operating instructions”.

The Independent has contacted Babcock for comment.