The £7m pumped into restoring Britain’s only WW2-era submarine reaped instant rewards as HMS Alliance doubled visitor numbers in a year.

More than 100,000 people have toured the new-look boat – and the rest of the Royal Navy Submarine Museum – in Gosport, up from just 43,000 visitors in 2013.

Alliance re-opened in April 2014 – and was rededicated the following month by the Duke of Cambridge, honorary head of the Silent Service and patron of the appeal to help bring the A-class boat back to the state she was when she entered service just after the end of World War 2.

When the two-and-a-half-year restoration project was completed, bosses at the National Museum of the Royal Navy – the umbrella for the numerous Naval Service museums – predicted a 20 per cent in numbers.

Alliance was built for action in the Pacific, but war in the Far East ended before she was finished.

Instead the boat had a distinguished three-decade career as a Cold War warrior before being raised on to cradles in the early 80s to form the centrepiece of the museum.

Thirty years later and the boat was in desperate need of restoration – carried out with a mix of lottery cash and fund-raising/donations.

As well as making the 281ft submarine look pristine outwardly, her innards were given a new look with her attack periscope restored to work order and sections of the boat recreating the periods in which she served.