Eurotunnel rose the most in a month in Paris trading after the company, which is based in the French capital, said that the U.S. bank’s infrastructure fund will exercise all of its certificates to take a 21.2 percent stake.

Goldman’s move is a “powerful sign of renewed confidence in the group,” Eurotunnel Chief Executive Officer Jacques Gounon said in a statement. “It will be a real asset in the pursuit of our upcoming strategic developments.”

Eurotunnel, which runs vehicle-carrying shuttles through the tunnel and sells access to Eurostar Group passenger trains and freight operators, recorded its first profit in 2007 after a reorganization slashed debt by 3 billion pounds ($5 billion). The company plans to expand by acquiring other European rail concessions, starting with the High Speed 1 link from London to the tunnel, earmarked for sale by the U.K. government.

Eurotunnel jumped 33 cents, or 7.8 percent, to 4.56 euros, the biggest gain since Aug. 7, extending the stock’s gain this year to 18 percent.

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Journey times from London to Paris and Brussels were cut by 20 minutes after the final section of High Speed 1 opened in 2007. The line’s U.K. terminus also moved north to St. Pancras station from Waterloo, making connections easier. The improvements helped Eurotunnel increase profit in the first half of 2008, before the recession and a tunnel fire dragged it back to an 8 million-euro ($11.5 million) loss a year later.

The Goldman fund’s future status as the main Eurotunnel shareholder became clear after it bought 82 percent of a placement of 800,000 subordinated deferred equity securities that it underwrote last year. Its stake is set to be diluted to 15.2 percent with the conversion of outstanding Eurotunnel securities and certificates to stock, the company said.

Goldman spokeswoman Joanna Carss declined to comment on the company’s strategy as a Eurotunnel investor.

The New York-based bank raised financing for the $6.5 billion infrastructure fund in 2006 to pursue large-scale investment opportunities in industries spanning toll roads, airports, ports and utilities. Goldman put up $750 million of the fund’s capital, with the rest coming mainly from pension funds, insurers and other banks.