An Italian TV reporter said Beckham told the physios: Its broken, its broken as he was taken off on a stretcher in tears.
He was preparing to kick a ball unchallenged when he frantically signalled to the bench something was wrong, The Sun reports.
He will have an operation in Finland tomorrow and faces three to four months of rehabilitation, ruling him out of the 2010 World Cup in South Africa.
Milan coach Leonardo said: Beckhams injury makes us feel terrible. He is an extraordinary guy and todays game proves it yet again.
I cannot enjoy this evening considering this serious injury. David understood immediately that he had torn his Achilles tendon, he said.
Three Lions boss Fabio Capell, who called Beckham after the game, now needs a serious rethink, with Becks rival for the right midfield berth, Aaron Lennon, also struggling with a groin problem.
And left-back Ashley Cole is in a race against time to make the finals as he recovers from a broken ankle.
