Hopes that Scotland's second emergency tug could return following the grounding of a 17,000-tonne oil rig in the Western Isles have been dashed.

The Orkney-based Herakles has been Scotland's only emergency towing vessel (ETV) since the Stornoway tug was scrapped five years ago.

Repeated calls have been made for the reintroduction of a second tug in the Hebrides since the Transocean Winner ran aground on Lewis last month.

The Maritime and Coastguard Agency (MCA) has now ruled out any prospect of a second Scottish ETV but suggested that the Herakles could be moved from Kirkwall in future.

It said it is likely the vessel will patrol Scottish waters, rather than being based permanently in Kirkwall.

An MCA spokeswoman said: "We consulted with a wide range of Scottish stakeholders during the early part of this year and commissioned an independent report to establish the emergency towing vessel requirement off north and north west Scotland.

"The case for providing two ETVs rather than one was considered during discussions.

"The operational experience of the past five years has demonstrated that a single ETV has been sufficient to meet the needs for emergency towage."

She added: "Given this evidence, it has been decided that a second ETV - delivered at taxpayer expense - is not warranted.

"The MCA will monitor the prevailing conditions and when appropriate will consider moving the ETV to a position within its overall operating area where the potential for increased risk has been identified.

"The new ETV contract is likely to include a patrolling function which should see visits to all parts of the ETV operating area having due regard to weather forecasts and vessel traffic service requirements."

Calls for a second emergency tug have been endorsed by the Western Isles and Highland councils.

Earlier this month, ten SNP politicians signed a letter warning the consequences of not having a tug in Stornoway could be "catastrophic".

Western Isles SNP MSP Alasdair Allan said: "The sad reality is that when the oil rig went aground on Lewis, the Orkney tug was roughly 18 hours away from the scene.

"The coastline of Scotland is massive and one tug simply cannot cover all of it.

"This is certainly a disappointment but it will not make the campaign for a second vessel go away.

"The people in my constituency are known for their knowledge of the sea and they do not regard this as a safe situation."

Emergency towing vessels were first introduced in 1994 following the Braer oil spill in Shetland.

The MV Braer spilled 84,500 tons of crude oil into the ocean when it went adrift and ran aground on January 10, 1993.

A report published in the wake of the disaster called for the introduction of a fleet of ETVs which could rescue a drifting vessel before it foundered.

Four ETVs were based at ports across the UK until 2011 after funding was slashed in the UK Government's 2010 spending review.