Islanders defending the last bastion of the Sabbatarian tradition claim to have won thousands of supporters as they face a new threat of Sunday ferry sailings.

Currently, 3760 people - equivalent to one in six of the combined population of Lewis and Harris - have signed a petition to register their strong opposition to any Sunday ferry service to Lewis, and returns are still coming in.

But the board of the publicly owned ferry company Caledonian MacBrayne meets on Wednesday to consider "thousands" of requests for a seven-day service on its Stornoway to Ullapool route.

Last year, CalMac angered many islanders by introducing a service between Berneray and Leverburgh in the south Harris which has similar traditions of Sabbath observance to Lewis. The most populous of the Hebrides, Lewis is now the only island not to enjoy a seven-day service because of religious tradition.

The Rev Andrew Coghill, the Church of Scotland minister at Leurbost, spokesman for the Lewis branch of the Lord's Day Observance Society (LDOS), said the response to the group's petition was very encouraging given the limited time.

"We are very grateful for the support we have received as it shows the strength of the feeling of the residents who are opposed to Sunday sailings. We did have to strike a number of names off the petition because they lived outside the island.

"There is no doubt in our minds that if a Sunday ferry service was introduced it would lead to demands for other facilities to be opened.

"Our Sabbath has contributed to a safe, peaceful environment, free from secular pressures, and is appreciated by locals and visitors, churchgoers and non-churchgoers alike."

The LDOS hopes to meet Stewart Stevenson, the Transport Secretary, to discuss the issue.

Unlike in previous years, government ministers will no longer be able to simply remain aloof from the issue because of the terms of the contract just signed with CalMac, after its routes were put out to tender.

It states: "Major adjustments (to the timetable) shall not be made without the consent in writing of the Scottish Ministers "

CalMac says it has had thousands of representations for and against a Sunday sailing and that is why the issue is on Wednesday's agenda.

Industry sources suggest that CalMac's board will have to postpone a decision until it is known whether the Ullapool/Stornoway route will be involved in the Scottish Government's pilot study of road equivalent tariff (RET), which should be revealed within the next few weeks.

In August, it was announced that at least one of the Western Isles' routes would act as an RET pilot. RET should mean the ferry fare is no greater than the cost of driving a car the same distance with £5m, £7.5m and £10m earmarked over the next three years in the Scottish Government's recent Budget.

It costs £229 for a car with driver and three passengers to make a return trip between Ullapool and Stornoway, a total journey of about 100 miles. Some claim that RET would cut that to around £30, though details of the RET equation have yet to be established. The service is running at close to full capacity already so it will not be possible to tell whether the RET pilot increases traffic unless there are extra services on Sundays.