Williamson began his career as a trainee with Torquay United before joining Southampton in 2001.
After failing to break into the first-team at St Mary's, he moved to Adams Park in May 2005 and went on to make more than 150 appearances for Wanderers, playing 21 times this season.
League One side Colchester - managed by ex-Wycombe boss Paul Lambert - had a bid for the player rejected earlier this month.
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