Ferry operator Stena Line has unveiled a £70m investment in the crossing between Scotland and Northern Ireland.
The firm currently sails from Stranraer
The firm announced that it will build new terminals at Cairnryan and Belfast - cutting 30 minutes off the journey time.
The investment was welcomed by First Minister Jack McConnell, who also announced a £14m package of road improvements for the A77 in south west Scotland.
But there was a mixed reaction in Stranraer, which will cease to be a ferry port for the first time in nearly 150 years.
Stena said it would move its Scottish terminal from Stranraer to Old House Point at Cairnryan.
The Belfast terminal will be relocated to the mouth of the port.
This will cut journey times on the HSS fast ferry to 80 minutes and allow the number of daily sailings to increase from 14 to 18.
The aim is to have the terminals completed by mid 2005.
Stena's chairman Dan Sten Olsen said the company had already spent £90m on the route in the past six years.
These investments will both safeguard around 400 high quality jobs and improve the transport infrastructure substantially
"Today's additional £70m investment is a further commitment to the route," he said.
Mr McConnell said the package of road improvements would improve access to south west Scotland for tourists and businesses.
"Stranraer serves as a gateway to Scotland and Stena's long term commitment here today will be vital for the south west economy and for Scotland as a whole," he said.
"The company is confident in its future and this provides long term security to its workforce and the businesses who depend on it.
"The roads investment I have confirmed today will provide valuable support for the signs of growth that we are beginning to see in this part of Scotland."
David Trimble, Northern Ireland's former first minister, said he was happy the new terminal was being built in Belfast.
He added: "Seeing the quality and frequency of the service improve is very important to us."
Scottish Enterprise Dumfries and Galloway has contributed £2m to develop Loch Ryan as a national transport and destination gateway.
Chief executive Colin Williamson said: "These investments will both safeguard around 400 high quality jobs and improve the transport infrastructure substantially.
We see it as an opportunity for the town to become more attractive
"In the longer term this will lead to both the regeneration of the Stranraer waterfront and will provide a significant boost to the wider economy of Dumfries and Galloway."
Former Stranraer councillor Donnie Nelson, the director of an economic development initiative, said Stena would leave "a big gap" when it left the town.
But he said he was "delighted" that so much money was coming into the area.
Stena had warned that it may quit the route altogether because of poor road links to and from the port.
Competitor Sea Containers relocated its SeaCat base from Stranraer to Troon for that reason.
Stranraer hotelier Douglas McMillan said it was "a big relief" that Stena was not quitting the area.
And he said: "We see it as an opportunity for the town to become more attractive at the front.
"If we are able to get some very good signage from both directions to the port that would be a bonus.
"If we can also divert a lot of the heavy traffic away from the town that will help, so there is a good side to it as well."