WASHINGTON, June 20 (Reuters) - Delta Air Lines Inc won European and U.S. antitrust approval to buy a 49 percent stake in British airline Virgin Atlantic, antitrust enforcers in Brussels and Washington said on Thursday. The U.S. Justice Department and the European Commission, which is Europe's competition watchdog, both cleared the proposed transaction. The U.S. Transportation Department must also approve the joint venture. The partnership would let both carriers expand at Heathrow, where constraints on landing slots have limited growth. It also gives Delta the firepower to attract prized U.S. corporate travelers as it competes with industry leader United Continental and others. Delta announced the deal in December, saying the purchase would allow the U.S. carrier to expand its access to London's Heathrow Airport. "After a thorough investigation of the competitive effects of the proposed equity investment and joint venture, the Antitrust Division concluded that the facts and circumstances did not warrant further investigation or action," the Justice Department said in a statement. Delta and Virgin would cooperate on frequent-flier programs and share travel lounge amenities. Heathrow, Europe's busiest airport, is operating at close to full capacity after Britain's coalition government blocked its expansion in 2010. Virgin's chief executive, Richard Branson, has said he will keep his 51 percent stake in Virgin Atlantic.