Ministry of Defence officials who have assessed the suitability of Gibralter estimate that¬†relocation would cost Britain about £3 billion and take up to 10 years to complete, the Sunday Express has reported.

A senior academic said that the MoD was clearly "reading the writing on¬†the wall" about the ≠possibility of another Scottish referendum.¬†SNP deputy leader Stewart Hosie has threatened to hold Westminster to ≠ransom over¬†Trident if it gains enough seats after May to hold the balance of power.

A senior military source said : "A party was sent to Gibraltar in January¬†to determine whether the option could work. One can only assume it's part of¬†broader ≠contingency planning."

Gibraltar has the ≠capacity to accommodate ≠submarines large enough for Trident. The move would prove more politically palatable than other UK mainland options¬†such as Falmouth.

Its deep Mediterranean waters would allow easy access, in contrast to ≠Devonport, which has only a single deep water channel large enough which would be vulnerable to sabotage. ¬†In addition, its 34 miles of excavated tunnels include enormous caverns that have been made blast proof against nuclear detonations and could be adapted to store nuclear warheads. Theodore Bromund, of the Washington DC-based Heritage Foundation think-tank said: "Gibraltar is already a first strike target, so moving Trident there would not increase concerns "There is no other available port in the UK. Gibraltar would require expansion rather than starting again."

University of York international ≠security lecturer Dr Nick Ritchie, said: "If the¬†MoD is exploring other options, it shows admirable planning and foresight.¬†Choosing Gibraltar would avoid the nimbyism question. Any alter≠native site in¬†England or Wales would carry political down sides. But Gibraltar has enough¬†patriotism to overcome this.¬†¬†"It would be interesting to see Spain's reaction, ≠however, since it is a non-nuclear member of Nato."

Gibraltar's chief minister Fabian Picardo said: "Gibraltar has had ≠facilities for¬†nuclear submarines for many years.¬†There are regular visits by British and American nuclear submarines. Gibraltar¬†has always loyally served the defence interests of the United Kingdom."

The Sunday Express revealed in September plans to temporarily relocate Trident to the US naval submarine base at Kings Bay, Georgia, in the event of victory by Scottish nationalists in the first or a future referendum. The MoD said: "We are fully committed to retaining the deterrent on the Clyde and indeed we are basing all our submarines there from 2020."