Islands to receive fuel price cut
Chief Secretary to the Treasury Danny Alexander said the initiative, which will reduce the cost of a litre of petrol or diesel by 5p, would "make a real difference to people who face the highest fuel costs in the country".
However concerns have been expressed that recent increases in the cost of fuel will limit its impact.
Alistair Carmichael, the Lib Dem MP who represents Orkney and Shetland, told BBC Radio Scotland he had received evidence from Shetland that "a local garage had had the price they were required to pay put up 2p per litre this week and 3p per litre last week".
When asked if that meant there would be no difference in the price paid by motorists, he said: "That's the danger."
However he insisted the scheme would "have an impact for people as long as everybody plays fairly and honourably".
Angus MacNeil, the SNP MP for the Western Isles, said fuel prices had already "increased by the equivalent of the discount over recent weeks". He added that the average cost for petrol and diesel had gone up by almost 20p a litre "since Danny Alexander first mooted the idea of a discount scheme".
The Nationalist argued: "There are clearly real questions over whether people will feel the benefit of the discount."
Mr MacNeil said: "It is simply unacceptable that in energy-rich Scotland, motorists and businesses are struggling to cope with prices at the pump. These record fuel costs are putting a huge and unnecessary amount of pressure on our communities and on vital sectors of the Scottish economy, and risk choking the recovery we are building in Scotland."