Plans for a subsea electricity cable linking Shetland to the mainland have been provisionally approved by the energy regulator.

Ofgem said it is minded to approve the Scottish and Southern Energy Networks' (SSEN) 600MW transmission link, which would allow new wind farms on Shetland to export renewable electricity to the rest of the UK and help ensure security of supply on the islands.

However it said it is minded to reject a similar project in the Western Isles.

The Shetland project would consist of a single 600MW subsea circuit from Kergord on Shetland to Noss Head in Caithness, connecting into SSEN's recently completed Caithness-Moray transmission link.

It is estimated the link would cost around £709m and would be completed in 2024.

Ofgem is now consulting on approving the link subject to SSEN demonstrating, by the end of 2019, that the Viking Energy Wind Farm project planned for Shetland has been awarded subsidies through the UK Government's Contracts for Difference (CfD) auction.

Colin Nicol, managing director of SSEN, said: "Ofgem's decision to provisionally approve SSEN's 600MW proposed transmission link is an important milestone in connecting Shetland to the mainland electricity network and helping to unlock its significant renewables potential.

"There are still a number of steps to be taken before we can proceed with the transmission connection, in particular the requirement for renewable developments to secure CfD support in this year's auction, and we will continue to engage with key stakeholders during this consultation period." However, the energy regulator has suggested changes to SSEN plans for the Western Isles.

In a statement, Ofgem said: "Ofgem would instead support an alternative proposal that more appropriately protects consumers from the additional costs of funding a potentially significantly underutilised link."

An initial estimate for the 600MW link for the Western Isles put the cost around £663m and a 450MW link at around £617m.

SSEN is calling on Ofgem to reconsider its provisional decision and said that a 450MW link would limit the potential for community schemes to benefit from renewables expansion.

Mr Nicol said: "Moving to a 450MW at this late stage also introduces risks and uncertainty which, in turn, could impact on the delivery of a transmission link to the Western Isles."