The company suggested over the weekend that “tourist travel” would return on Thursday, 14 April.

But on Monday it tweeted some sailings have resumed, with departures from Cairnryan at 04:00 BST, 12:00 and 20:00 and Larne at 08:00, 16:00 and 23:59.

The European Causeway boat used on the service was detained in Larne last month over safety concerns.

That followed the company sacking 800 staff without notice and replacing them with cheaper agency workers.

There was confusion over the weekend when P&O initially tweeted that passengers could come on board from Sunday, but that changed to Wednesday before Thursday was suggested as a likely start date.

The operator’s Dover-Calais service is expected to resume on that date. P&O said on Monday sailings “remain cancelled”.

On Saturday the European Causeway sailed, with limited services, on a return journey between Larne and Cairnryan.

It was the first time the vessel had been allowed to sail since it was detained in Larne a fortnight ago, but only freight was transported on the vessel. It is now understood foot passengers are allowed to travel again.

The RMT union has said it would attempt to blockade ports, including Cairnryan, in protest when the ferry operator started running what the union labelled its “ships of shame” again.

Regional organiser Gordon Martin branded the company “capitalist bandits” for the way in which it sacked staff with no notice via video call.

“We will be after you, we will be coming to Cairnryan, Larne, and elsewhere and we will be targeting your supply chain,” he told BBC Scotland.

“Anybody dealing with P&O – I’ll send a clear message to them – whether it be fuel providers, food providers, whatever it may be.

“We will find out who you are and you will be part of our efforts to get proper legislation in place to protect workers.”

The European Causeway was initially detained in Larne with the Maritime and Coastguard Agency (MCA) concerned about “failures on crew familiarisation, vessel documentation and crew training”.

Stena Line have put on extra ferries from Cairnryan to Belfast to compensate for the postponement of P&O services.

The operator increased its daily sailings on the route from 12 to 18 and said its staff had been working hard to meet demand.

The Scottish government condemned the “serious disruption” P&O’s actions had caused to the supply chain between Scotland and Northern Ireland.

A spokesman said: “The employment practices P&O have needlessly pursued have no place in a modern working practices.

“The Scottish government has been absolutely clear that we will stand by the workers who have been let down in the most appalling manner imaginable by P&O.”

He added ministers were seeking urgent meetings with the UK government to discuss the matter.

A spokesman for the UK Department for Business, Energy and Industrial Strategy said: “The Insolvency Service has confirmed it has commenced formal criminal and civil investigations into the circumstances surrounding the recent appalling behaviour of P&O Ferries, following the Business Secretary asking they undertake a thorough review.

“We will not provide further comment while investigations are ongoing.”

P&O has been approached for comment.