BT this week announced it has completed the deployment of a 250 kilometre submarine cable system that will help to bring high-speed broadband connectivity to the U.K.'s Highlands and Islands region.

The cable system links 40 off-shore and mainland Scotland locations. It forms part of Digital Scotland Superfast Broadband's £410 million project to upgrade Scotland's broadband infrastructure. Of that sum, £146 million is being ploughed into improving connectivity in the Highlands and Islands region, which includes the £26.9 million subsea network.

"Today marks an incredibly important step in the completion of the most complex ever underwater engineering that Scotland has seen," declared Scotland's deputy first minister John Swinney, in a statement on Thursday.

The deployment involved 20 seabed crossings, over distances ranging from one mile to 50 miles.

"This has been the most complex subsea project BT has tackled in U.K. waters, as well as being the largest number of seabed cables laid in a single weather window. I'm so proud of the endeavour shown by the whole team to overcome the technical and environmental challenges," said Brendan Dick, director of BT Scotland.

"This underwater spider's web of fibre optic cables is set to deliver a seismic shift in communications for Scotland's island communities, bringing them in closer touch with the rest of the world than ever before," he said.

The next step will be to upgrade the region's terrestrial networks. The bulk of the network will consist of fibre-to-the-cabinet (FTTC); BT said it also expects to roll out some fibre-to-the-premises (FTTP).

The Highlands and Islands project aims to deliver high-speed broadband to 84% of the Highlands and Islands by the end of 2016.