The Scottish government has come under heavy criticism for failing to offer fair funding for Shetland’s inter-island links.
Finance secretary Derek Mackay had “ignored the express will of Parliament” during his big budget speech at Holyrood, according to MSP Tavish Scott.
A heavy defeat was suffered by the SNP last week, when the administration was challenged to honour its long-standing ferry funding commitment.
But there were was no commitment to ferries when Mr Mackay set out his spending and tax plans, which will see Scots earning more than £33,000 hit by income tax rises next year.
The Liberal Democrats have now announced proposals to bring an amendment that will encourage the SNP administration to restate its support for ferry services.
Earlier in the day Orkney MSP Liam McArthur urged the First Minister to step in and ensure her government delivered on its promise of fair ferry funding.
In a joint statement following the budget, Messrs McArthur and Scott said: “The decision by this government to abandon the promises it made to the communities in Orkney and Shetland is reprehensible.
“Having committed to fair funding for internal ferry services in the Northern Isles back in 2014, the finance secretary has chosen to leave the communities who rely on these lifeline services high and dry.
“Mr Mackay and this SNP government have also shown contempt for the will of parliament.
“We will now need consider the detail of the government’s budget over the coming weeks. However, we are determined to bring forward an amendment ahead of the vote next month that will allow parliament to restate its support for fair funding for our internal ferry services. This is something we will be discussing with the other parties in the run up to that vote.
“In the meantime, Mr Mackay has done himself and his government no favours by abandoning his promises and ignoring parliament. This will not have gone unnoticed in the communities left facing the consequences of his decision.”