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Loosehead is among the more competitive positions within the England ranks but with Alex Corbisiero suffering with long-term injury problems, Marler has established himself as the first choice, with Mako Vunipola supporting him from the bench.
Marler was one of four Harlequins players to impress for England during the RBS 6 Nations, along with captain Chris Robshaw, scrum-half Danny Care and RBS Player of the Championship Mike Brown.
And after extending his contract until 2016, Marler is only going to get better according to Harlequins director of rugby O'Shea.
"Joe is and will continue to be a cornerstone in the pack and in the success of this club," said O'Shea.
"He's developing into one of the leading looseheads in international rugby."