Fuel campaigners in the Western Isles have compiled a dossier of what they claim is evidence of discriminatory behaviour by the area's main fuel distributors.

They have sent a copy to Danny Alexander, the Chief Secretary to the Treasury, following his launch on Thursday of the 5p fuel duty discount scheme for islanders.

There have been increasing calls for an investigation into the charges levied by Scottish Fuels, part of GB Oils, as recent price increases appear to cancel out any benefit from Mr Alexander's scheme.

Fair Fuels Solutions spokesman Frank Burns said of the price rise: "The company that supplies fuel to the islands is GB Oils Ltd. It has a partnership arrangement with ConocoPhillips. This week they deliberately reduced what should have been a significant move forward for the people of the islands into what amounts to humiliation of a senior government minister."

In its letter to Mr Alexander, the campaign group said that on Thursday, a litre of unleaded petrol on Skye was 133.9p, while in Stornoway – less than 40 miles away but on the other side of the Minch – it was 147.9p.

Sam Chambers, director of GB Oils and Scottish Fuels, has dismissed any suggestion of profiteering.