A fraud investigation is underway involving claims that temporary passes were misused at the Royal Navy's nuclear submarine base at Devonport.

The inquiry centres on a claim for payment made by a private company and involves the use of passes to the dry dock built to refit Trident submarines.

The firm that manages the site said security at the Devon base had not been affected by the alleged fraud.

The investigation comes after the National Audit Office began an inquiry into the rising cost of the dry dock, which could cost 50% more than the �417m estimate.

Work on the dockyard has taken five years

The fraud investigation - being conducted by Ministry of Defence police - focuses on number nine dock, built to refit Trident nuclear submarines.

The investigation began in February and centres on the dry dock built to house HMS Vanguard.

Devonport Management Limited, the private company managing the site, said: "We aren't aware of any direct security implications associated with the investigation.

"During the period concerned the affected part of the dockyard was a major construction site and wasn't involved in submarine support or refitting activities."

Number Nine dock, built at the turn of the last century, had to be lined with massive earthquake-proof pillars and 50 "seismically-hardened" cradle supports to come up to standard.

The new complex has taken five years work and involving 40,000 people.

The 16,000-tonne HMS Vanguard became the first submarine to use the new facilities when it arrived in Plymouth in March.