The biggest joint-marketing deal in the history of sport, potentially worth billions of pounds, was unveiled at a press conference in Manhattan on Wednesday.

But United officials insist the deal does not mean they are looking to abandon the Premiership.

Paddy Harverson, United's director of communications, said: "Manchester United are fully committed to the Premier League and domestic English football.

"There is no suggestion whatsoever that we are looking to leave the English game and join any kind of breakaway league."

The teams will share market information, develop sponsorship and joint promotional programs and sell each other's licensed goods.

"The YankeeNets have interests in baseball, basketball, ice hockey and a marketing partner of the New York Giants," said United chief executive Peter Kenyon.

"They have a particularly well developed multi-media business."

Peter Draper, United's group marketing director, will work with John Krimsky, president of YankeeNets Properties, to implement the new initiative.

The Yankees will use United's own vast network in Europe and the Far East to sell their distinctive merchandise.

Under the deal, David Beckham and Fabien Barthez will be paraded at the Yankee Stadium, with El Duque and outfielder Bernie Williams visiting Old Trafford.

United will also embark on a pre-season tour of North America in 2003.

Shares in the Premiership club went up 20p to 233¾p - an increase of around nine per cent - after news of the partnership leaked out.

They are already the richest football club in the world with revenue that dwarfs the rest of the Premiership and major European clubs like Barcelona, Real Madrid and Juventus.