Irish Ferries has blamed unofficial action by up to 70 protesters for preventing its MV Normandyship from docking at Cherbourg in France.
MV Normandyclaims it has been prevented from docking at the French port because of picketing by Irish, British and French union officials, resulting in the stranding of nearly 800 passengers on board.
But unions denied the claims by Irish Ferries that they are blocking the ferry from docking at the port.
In a statement, the company said it believed the action "may relate to the decision by Irish Ferries to contract in EU crew to operate its French service".
The ship sailed from Rosslare yesterday afternoon and was due to dock at Cherbourg at about 10.30am.
There are almost 800 passengers on board, along with 250 cars and four freight trucks. The ship's captain said earlier it could not berth as there were up to 70 protesters on the ramp.
But International Transport Federation spokesman Tony Ayton insisted the protest was a peaceful one and there was no threat to the safety of passengers.
"It is absolutely ridiculous for Irish Ferries to use that excuse to deny the passengers their lawful right to come ashore," he said.
Mr Ayton admitted some protesters were on the ramp for a short time earlier this morning as a TV company wanted to record footage of them with the ship in the background.
He added: "We're here fighting for British, Irish and French seafarers and their right to support a family and pay a mortgage, as has been the long-standing tradition on ships operating on these waters."
Workers at Irish Ferries have complained about the alleged use cheap labour on the Normandy, claiming that eastern European workers are employed on the ferry at pay rates of €3.57 per hour.
They work 12-hour shifts, seven days a week with no holiday pay, it is understood. Earlier this year, it emerged a Filipino beautician working on another Irish Ferries route was paid one euro an hour. Her contract was later cancelled and the firm made a settlement with her.
Irish Ferries said today it should be noted that the company was in the process of implementing a Labour Court recommendation in respect of the Normandy which the company and Siptu (representing ships' officers) had both accepted.
"Moreover, Irish Ferries has assured the unions, via the Labour Court, that pay and conditions being implemented by the contractor, through whom the crew of the Normandy are employed, are equivalent to or better than International Transport Workers' Federation standards which apply to the company's major competitors," it added.
"The pay, terms and conditions being applied by DFM Ltd, ( Dobson Fleet Management ), the ship's management contractor, to their staff on the vessel are not only fully compliant with ITF but, in many cases, are well in excess of such pay, terms and conditions."
Irish Ferries said the ship was just five- to ten-minutes off docking when the protesters arrived.
It said between 60 and 70 protesters displaying the flag of Brittany Ferries, blocked the ramp "thereby preventing the vessel from docking in the normal manner".
"As a consequence, the vessel was prevented from discharging its 793 passengers, 248 cars, 4 articulated freight units and two vans that are still held on board. On the instructions of harbour authorities, the vessel is now waiting off the harbour entrance pending resolution of the issue."
Irish Ferries said it had been given neither formal nor informal notification of the action.
The vessel is due to sail again from Cherbourg to Rosslare at 6pm this evening. Some 658 passengers and 232 cars are booked to travel to Ireland and the ship is scheduled to arrive in Rosslare tomorrow morning at 11.30am.
"Irish Ferries deeply regrets this situation which is not of the company's making and the inconvenience which it is causing to passengers, all of whom are innocently bound up in this action."
Additional reporting PA