Bidding to become the first English player to appear in four World Cups, Beckham was injured without getting hit during AC Milan's 1-0 Italian League win over visiting Chievo Verona.
With only a few minutes remaining and the score 0-0, Beckham was by himself with the ball at his feet, shifted his weight and reached a hand down to his left heel. Beckham then stood up and snapped his wrists like he was breaking a twig in half in a gesture to show the AC Milan bench he knew the tendon was broken.
Visibly in pain and in tears, Beckham went to the sideline for medical attention, leaving Milan a man short because all three substitutes had been used.
"He felt the muscle begin to come up, which is a typical symptom when you break an Achilles' tendon," Milan coach Leonardo told Sky. "This is a real blow."
Sky reported that Beckham said, 'It's broken, it's broken," when he came off.
Club physician Jean Pierre Meersseman told Italy's Sky TV that Beckham will fly to Finland, where he will be treated by specialist surgeon Dr. Sakari Orava.
"He'll go to Finland tomorrow and will be operated on probably tomorrow afternoon or Tuesday morning," Meersseman said.
While Beckham has not been a starter for England in recent matches, he was likely to make the World Cup roster as a reserve. Although no longer a top player, he was still useful for his free kicks and crosses, especially when England needed second-half goals.
And for many, he was the most recognizable player in the world, a fashion icon with a celebrity wife, former Spice Girl Victoria Beckham.
The 34-year-old midfielder was on his second loan stint to AC Milan from Los Angeles Galaxy as he looked to boost his chances of making England coach Fabio Capello's World Cup squad. Instead, his international career is likely over after 17 goals and 115 appearances, second in English history behind only goalkeeper Peter Shilton's 125 appearances from 1970-90.
Beckham was England's captain from November 2000 through the 2006 World Cup. He was due back with the Galaxy after the World Cup.
"He'll probably be out for five or six months," AC Milan vice president Adriano Galliani told Sky. "I saw him really suffering. In the changing room I hugged him and told him that if he wants he can join us next year, too."
It was yet another blow for Major League Soccer, already facing the threat of a players' strike ahead of the season opener on March 25. Beckham will miss most or all of the 2010 Galaxy season.
Returning to the starting lineup after Milan's 4-0 Champions League loss to Manchester United, Beckham was also treated for a deep cut to his right cheek following a collision early in the first half.
With the game scoreless, Beckham nearly scored in the 84th, but Chievo goalkeeper Stefano Sorrentino stopped his close-range effort.
"Beckham came here because he wanted to help Milan return to the top and he was looking to get a jersey for the national team and participate in the World Cup," Milan defender Gianluca Zambrotta said. "If other victories come, there will surely be a dedication for David Beckham."
Sunday's win moved Milan (17-4-7) within one point of Serie A leader Inter Milan (17-3-8) with 10 games remaining.