Progress on reversing the depopulation of Gigha since a community buyout six years ago could be jeopardised if the authorities cannot secure the ferry service to the mainland, according to the chairman of the island's heritage trust.

For weeks, Caledonian MacBrayne's service between Gigha and Tayinloan on the Kintyre peninsula has been disrupted at low tide because of a build-up of silt and weeds at the mainland slipway.

As a result, instead of the regular 20-minute crossing, the 150 islanders have to endure a sail of up to two hours to Kennacraig in West Loch Tarbert.

Yesterday, Willie McSporran, chairman of the trust which owns the island where the population has increased by 50% since the buyout, said: "The problem is now getting critical. It affects everything. It can't go on."

Argyll and Bute Council has responsibility for the slipway and had commissioned a dredger to clear the sand and weed but, according to Mr McSporran, that would last only until the next storm. What is needed, he said, is an extension to the slipway, which the council claims it cannot afford.

A spokesman for the council said that the local area committee had visited the island this week.

"Grant funding for piers and harbours is no longer available from the Scottish Government; these monies have been disbursed to the five authorities in Scotland as part of their block allocation. The council remains absolutely committed to implementing a scheme which will satisfy the principal users of the ferry slip and the local landowners," he added.

A Scottish Government spokesman said: "Funding for any improvements is a matter for Argyll and Bute Council. However, the Scottish Government has included £1m in each of 2009/10 and 2010/11 towards this project."