The shipping company behind plans to buy routes and vessels from P&O will stop sailing from Flintshire after investing more than £12m in the port of Holyhead.
The Stena Adventurer will sail from Holyhead
Stena Line announced plans to buy P&O's routes and vessels at the ports of Liverpool, Fleetwood and Dublin on Tuesday.
However, the company stopped short of acquiring the P&O routes from Mostyn Docks in Flintshire, which had received a large investment just 18 months ago.
The company, which will also buy the two ferries which operate out of Mostyn, said it would not be "economically viable" to run two rival services from north Wales.
Passengers wanting to travel from north east Wales will be forced to travel to Anglesey once the deal has been completed in six months time.
Stena clearly cannot allow Mostyn to continue because it competes with Holyhead
Patrick Heesom Mostyn councillor
Stena spokesman Joakim Kenndal said it would not make good business sense to pit two ports which are less than 100 miles apart against each other.
"The Mostyn route just doesn't fit in with our strategy, we already have a route from Holyhead to Dublin so we can't compete with ourselves," he said.
"We have never been interested in Mostyn as part of this deal.
"We are rebuilding Holyhead port and bringing a new ship in, there is no reason to develop a new port - it would not be economically viable."
Flintshire councillor for Mostyn Patrick Heesom has written to First Minister Rhodri Morgan asking him to intervene.
"Mostyn has been caught between two multi-national ferry companies," he said.
Mostyn Docks will continue to trade
"Stena clearly cannot allow Mostyn to continue because it competes with Holyhead," he said.
"This is a classic case of commercial interest walking all over public interest."
Stena Line is increasing its capacity from north west Wales.
The Stena Adventurer, which can carry 1,500 passengers, has been built in the Far East by Hyundai Heavy Industries and is due to start operating on the Holyhead to Dublin route in July.
P&O pledged their support to Mostyn Docks just 18 months ago.
The port was officially reopened with a terminal for transport freight and car passengers in February 2001 following a £17m facelift.
However, P&O's decision to pull out of the port has led to the redundancy of 26 of their staff.
The deal with Stena Line could take up to six months to compete.
Ferry services will continue as normal until that time.
Mostyn Docks will continue as a cargo operator and Jim O'Toole, the port's managing director, confirmed that as far as he was concerned it is "business as usual".