The development and construction of an undersea tunnel between mainland Shetland and the island of Yell would cost £402m and take eight years to complete, it has been estimated.

The projections have been published by the engineering consultancy firm COWI which has been working alongside Shetland Islands Council (SIC) and fellow consultants Stantec, says the BBC.

Yell is the test case in a project comparing the cost of future ferry upgrades and investment in fixed links, due to concern over ageing infrastructure. The initiative is at an early stage, with funding and a full business case still to be developed.

The 4.2 mile (6.8km) tunnel would be at a depth of 50m (165ft) below the seabed and include tolls to cover maintenance costs.

Andy Sloan, executive vice president of COWI, said the estimates were based on discussions with three international contractors.

“The capital cost investment is about £327m,” he said. On top of that we need to add risk and contingency money of £50m and upfront costs of £25m.”

The Yell link has been described as a “base case” which could be replicated for tunnels to three other islands: Bressay, Unst and Whalsay.

“While it seems like a big number, in a Scotland or UK-wide context it’s entirely reasonable”, said Sloan.

The Faroe Islands already have a network of tunnels linking them, four of them under the sea (pictured).

Proposed funding models include private investment, grant funding from the Scottish and UK governments, and local authority borrowing.

Neil Grant, a director with SIC, said both governments had shown interest, adding:

“The Scottish government, in particular, has been supportive of the work to date.”

However, he said “intense work” was required to reach the stage of a full business case.

(Picture: Ólavur Frederiksen/Faroephoto.com)