PORTLAND – A decades-old agreement that makes the Scotia Prince the only ferry to operate between Portland and Nova Scotia is coming under federal scrutiny.

The Federal Maritime Commission plans to review the contract that gives exclusive use of Portland’s International Marine Terminal to Scotia Prince Cruises Ltd.

Regulators said the contract with the city may restrict competition and violate U.S. maritime law.

“If you have an agreement with an exclusive arrangement which forecloses competition, we take a look at it,” said Bryant VanBrakle, commission secretary.

VanBrakle emphasized that the commission’s order to investigate possible anti-competition practices does not mean they exist, and that an exclusive arrangement with the ferry line could be justified.

The commission’s review will be watched closely by Bay Ferries Ltd., operator of The Cat, a high-speed ferry between Bar Harbor and Yarmouth, Nova Scotia. Bay Ferries has expressed interest in expanding service on The Cat to Portland.

In sending the Scotia Prince contract to the commission, the city was seeking guidance in writing a contract for the Ocean Gateway international passenger terminal, which is set to open in 2006.

“Every indication I have is we have an agreement with Scotia Prince Cruises, and we are going to find out whether that is a valid agreement,” said Jeffrey Monroe, Portland’s director of ports and transportation.

A representative of Scotia Prince described the commission’s decision as a routine procedure, which the company is reviewing.