The 24-year-old, who who made his England debut against Trinidad and Tobago on Sunday, started discussions with the club when the Barclays Premier League season finished and signed the deal on Wednesday night.
Ashton told his club´s website: "I am delighted. I said all along that I wanted to sign. I feel I have still got a lot to show the football club and the fans. I am really happy that I have signed - and now I can look forward to next season."
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