World Champion Tom Boonen left the Tour on Tuesday, giving up his challenge for the green jersey he so badly wanted. The Belgian had trouble with his breathing and quit the race at the feed in Serre Chevalier, 110km in to the stage.

Despite holding the maillot jaune for four stages, Boonen wasn?t on the form that he showed last year, when he crashed out of the race while in green, but was still focused on winning a stage. So far he?d had no answer to Robbie McEwen?s blistering finishes, but badly wanted to redress the balance by taking victory on the Champs Elysees, where he won in 2004.

Boonen?s absence virtually guarantees Australian McEwen will win his third green jersey in Paris on Sunday.

Boonen first noticed a problem with his breathing on the stage to Carcassonne. He survived the stage to Gap but the problem returned on today?s stage when the pace increased on the Izoard.

Boonen had suffered with stomach problems earlier in the race but continued through. With two more tough stages in the Alpes, and continuous hot, muggy conditions he decided enough was enough.

?I?m really disappointed, especially for the team who?ve worked so hard for me,? he said after the stage.

The world champ will now return home and undergo medical checks. His program for the rest of the season will depend on those results.