The move has left council leaders disappointed
Caledonian MacBrayne will introduce Sunday ferry services from the mainland to Stornoway on Lewis in the Western Isles, the operator has confirmed.
Opponents Comhairle nan Eilean Siar and the Lord's Day Observance Society have been notified of the decision.
The local authority and the society have been strongly opposed to the the launch of the service.
The comhairle said it was disappointed but not surprised and will ask the Scottish Government to intervene.
The first sailing will take place on Sunday, following the Hebridean Celtic Festival which runs from Wednesday to Saturday.
There is currently no direct ferry service across The Minch from the mainland to Lewis and Harris on a Sunday.
CalMac said the decision followed extensive consultation.
The new service will initially follow the timetable of the Saturday afternoon sailing - departing Stornoway at 1430 BST to arrive in Ullapool at 1730 BST then leaving at 1815 BST arriving back in Stornoway at 2100 BST.
Chairman Peter Timms said the company had tried to find a way to meet its legal obligations and respect local traditions.
He said: "We believe we can achieve that by operating one return trip a day, departing in the afternoon and returning late in the evening.
"This will minimise the impact on the culture and amenity of the islands, while at the same time providing economic and social benefits."
Mr Timms added: "We remain acutely aware of the sensitivities surrounding Sunday sailings, but we cannot operate unlawfully nor fail to provide lifeline services when there is a growing demonstrable demand from the communities we serve."
The comhairle ruling administration said it was dismayed by the decision to sail between Stornoway and Ullapool.
Leader Angus Campbell said: "This is not a matter of surprise but it is still a disappointment.
"I am particularly unhappy that Caledonian MacBrayne have refused to meaningfully consult with the wider comhairle, nor have they provided us with the legal advice that they have based the decision upon."
In May, BBC Alba learned that state-owned CalMac viewed Sunday sailings to and from the mainland as "inevitable".
At the time the ferry operator said it had been told it would be unlawful to refuse to run a service because of the religious views of just part of a community.
Pro-sailings campaigners sought advice from the Equality and Human Rights Commission.
The Lord's Day Observance Society and Western Isles council leaders have previously stressed their opposition to the plans.
The comhairle said its policy was to reflect the customs and traditions of Lewis and Harris.