The petition is the work of Clydebank and Milngavie MSP Des McNulty and his Highlands and Islands colleague David Stewart, and numbers six independent or Liberal Democrat councillors in Argyll and Bute, including Bute's Len Scoullar, among its 67 signatories as this story goes live on Wednesday afternoon.

The petition criticises the minority SNP Scottish Government for the limited scope of its current trial of Road Equivalent Tariff (RET), which applies only to selected routes in the Western Isles.

Cllr Scoullar's signature is accompanied by the statement: "This RET pilot is discriminatory and in fact favours the islands involved in the pilot against the rest of Scotland's islands."

Neither of Cllr Scoullar's fellow Bute representatives, SNP councillors Robert Macintyre and Isobel Strong, have added their names to the document.

The petition is similar to one Argyll and Bute Council was planning to establish itself, but Cllr Scoullar said it had been decided to 'piggyback' on to the Labour MSPs' petition rather than create one which was separate but arguing for the same thing.

"At a meeting in Oban on April 19 council members wanted to support a petition calling for a similar scheme to the 40 per cent discount given to residents of islands with regular, scheduled air services," he said, "but we are now going to add our support to this existing petition instead.

"I would encourage everyone to access the petition and to sign it so that we can present our expression of support to the Scottish Government."

The petition's launch back in May was let down slightly by Mr McNulty's statement that the Largs-Cumbrae ferry route was the shortest one in the Clyde; critics also noted at the time that neither Mr McNulty nor Mr Stewart had led a campaign for lower ferry fares during Labour's eight years in power with the Liberal Democrats at Holyrood.

The Scottish Government's RET trial in the Western Isles will begin later this year.

* Find the petition online at www.ipetitions.com/petition/clydeandargyll.