Delta Air Lines is buying Singapore Airlines\' 49 percent stake in Virgin Atlantic Airways, forging a joint venture to expand the carriers\' trans-Atlantic network, the companies announced Tuesday. Under the agreement, Delta will invest $360 million and British mogul Richard Branson\'s Virgin Atlantic will keep the 51 percent controlling stake. Virgin Atlantic Airways will retain its brand and operating certificate. \"Our new partnership with Virgin Atlantic will strengthen both airlines and provide a more effective competitor between North America and the UK, particularly on the New York-London route, which is the largest airline route between the US and Europe,\" said Delta chief executive Richard Anderson. The alliance gives the Atlanta, Georgia-based Delta greater presence at the congested London Heathrow airport. The two carriers will operate as many as 31 round-trip flights between Britain and North America, 23 of which operate out of London-Heathrow.