The 22-year-old England international was allowed to travel to the London club for tests after Cardiff accepted an offer and he officially signed for the club on Tuesday.
"'Harry was a big factor for me, having worked with him at Tottenham.
"Being able to play with Rio Ferdinand was also a big draw. He has been a role model for many years, along with Ledley King.
"Our first aim this season has to be to get to 40 points, to make sure we stay up."
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