You would normally expect twins to speak the same language, but that was not the case at Clyde Naval Base in Scotland.

There, two submarines with an identical purpose and near-identical name, but separated by a narrow stretch of sea (the English Channel or La Manche, depending on your perspective) have ‘tied the knot’ in a unique twinning agreement.

As a further act of cementing the 100-year-old bond of Entente Cordiale between Britain and France, strategic missile submarines HMS Vigilant and her French counterpart FS Le Vigilant are now officially linked.

The crew of the French boat, built in Cherbourg and due to be declared operational later this year, and some of their country’s naval leaders gathered at Faslane to sign the agreement bonding the two boats.

But anyone expecting long-standing military co-operation between the two navies’ nuclear forces from this twinning will be disappointed.

The nature of the two nations’ nuclear deterrents means large sections of the two submarines are off limits to crew members of the twin boat if they are given tours.

And Vigilant and Le Vigilant will not be operating side-by-side, as nuclear ballistic missile submarines are by nature solitary beasts.

The last thing a strategic boat wants in her vicinity on patrol is another submarine out there, friendly or not.

“ This is much more about ‘hands across the Channel’,” explained Vigilant’s Executive Officer Lt Cdr ‘Rosco’ Tanner. “It’s not a case of military links so much as cultural ties.

“Obviously as submariners we would be very interested in looking around Le Vigilant. But I think it’s fair to say that the twinning has been a bit sensitive because of the secrecy surrounding nuclear boats.

“The centenary of the Entente Cordiale has helped it along.”

It’s not surprising the British submarine’s crew want to look around Le Vigilant – she’s the newest addition to the French nuclear-missile carrying submarine fleet, based in Brest.

There are also cultural differences between the two submarines. On Le Vigilant it’s not port and starboard crews, rather rouge and bleu – red and blue.

Some things, however, remain the same, including a love of football. The Entente Cordiale Trophy has been instituted to mark the twinning and will be fiercely contested.

The twinning ceremony was the highlight of a busy month for HMS Vigilant – she sent an honour guard to the opening of the Isle of Man’s Parliament on the same day as the French party arrived in Faslane, as the boat is affiliated to the island.

The two submarines are not dissimilar in size, with the British boats being slightly larger than her French twin. Vigilant, launched in 1995, is 150 metres long (Le Vigilant is 138m), the British boat displaces 16,000 tonnes dived (Le Vigilant 14,335 tonnes), and has a crew of 135 (there are 111 on Le Vigilant). Both can carry 16 nuclear missiles as well as torpedoes.