Second row George Robson will leave Harlequins at the end of the season to join French Top 14 side Oyonnax.
The 29-year-old has spent 11 seasons at The Stoop after progressing through the academy, making almost 200 appearances.
"I am extremely proud to have played a part in the success achieved by Harlequins in recent seasons," he said.
"However, I have decided that the time is now right for me to challenge myself in a different environment and culture."
Robson was a key part of the Harlequins side which won the European Challenge Cup in 2011 and the Premiership title a year later.
He has also represented England Saxons, and captained the England senior midweek side on the tour of South Africa in 2012.
Quins director of rugby Conor O'Shea described the lock forward as "one of the great servants of the club".
"We sat down last summer and George said he wanted to consider where he was before making any further commitments," said O'Shea.
"On consideration George told me he was looking to experience something different and a few years in France for himself and his wife was an opportunity too good for them to turn down.
"He goes with our best wishes. We will miss him hugely."