Confirming the news to the Official Club Website, Town Chairman David Sheepshanks said: "It is always sad to sell one of your home-grown players but this has become a necessity.
"Both the player and his agent have, for the last 12 months, declined to discuss any new long-term contract, other than if we were promoted.
"It is understandable that the player wishes to play in the Premier League, leaving us no alternative other than to realise maximum value through his transfer."
Charlton had a £3M bid for Bent rejected last summer by the club, something which Town Chairman David Sheepshanks admits was a gamble that in the end did not pay off.
"Last summer we didn't sell Darren, instead taking a calculated risk in retaining him in the hope that he would score the goals to take us to the Premiership. It very nearly worked - but not quite." he added.
Sheepshanks also felt Town were getting a good price: "With just one year left on a contract, negotiations are tougher.
"However, after carefully considering the offer, I, along with the manager and the board, are satisfied that while I think Charlton Athletic are getting a fine young player, we are getting full value in today's transfer market."
Sheepshanks closed by wishing Bent all the best in the future at his new club: "I would like to thank Darren for everything he has achieved with this Club and wish him every success for the future both with Charlton and hopefully England."
In addition to the fees, there is a sell-on clause for Town of a percentage that is not yet known.