OSLO (Bloomberg) — Norway's government breached the constitution when it changed fiscal rules for shipowners including Farstad Shipping ASA to tax income that had been exempt, the country's highest court ruled.

Bergshav Tankers AS and BW Gas Ltd. also were included in the decision, the Supreme Court said on Friday in a statement. Taxes paid by companies will be refunded with interest, according to Petter Narvestad, an analyst at Fondsfinans ASA in Oslo. Farstad jumped as much as 12 percent, the most in almost a year, in Oslo trading.

The state unveiled the changes, including the scrapping of a corporate tax on the industry, in 2007 in an effort to staunch a flow of new-vessel registries in countries such as Liberia and Bermuda with lower taxes. The new rules required shipowners to pay 21 billion kroner ($3.5 billion) of taxes on income that had been exempt for a decade.

"The government has suffered a major defeat," Narvestad wrote in a note yesterday. "The ruling means the companies involved will be able to reverse tax debt back to equity."

When the change was unveiled, Farstad said its tax bill would rise by about 1 billion kroner and that its future investment ability would drop by 5 billion kroner. BW Gas said it would owe as much as 4.8 billion kroner.