It had been rumoured that Crewe had been monitoring King last season and where pre-paired to move for the ex-Oxford United player during the summer as the Railwaymen looked to bolster their defence and push for promotion.
24-year-old King is in the last year of his contract and has already turned down a new improved contract at the club. Because of that the club had no other option but to place the defender on the transfer list.
Oxford born King has made over 150 appearances for The Bees since he joined them back in the summer of 2003 and was voted Player of the Year for 2006-07 after some sterling performance in the heart of the defence.