Hussain took 67 Championship wickets at 22.34 for Gloucestershire in 2010
Gemaal Hussain has left Gloucestershire following his side's failure to win promotion in the County Championship.
The county took a chance on Londoner Hussain when, following unsuccessful trials elsewhere, they handed him a two-year contract in May 2009.
He responded by ending 2010 as Division Two's leading wicket-taker, with 67 scalps in his first full season.
But chief executive Tom Richardson told BBC Radio Bristol: "He's off because he wants to play First Division cricket."
Hussain, 27, has been tipped for a move to new county champions Nottinghamshire, who have lost Ryan Sidebottom back to Yorkshire and have yet to persuade Charlie Shreck to sign a new deal.
But it is another big blow to Gloucestershire, who have already had their attack depleted by the departure of Steve Kirby to Somerset and Anthony Ireland to Middlesex.
I believe it will enable me to gain the experience and recognition necessary to be considered to play for England in the future
And Gloucestershire, who have now also lost opening batsman William Porterfield to Warwickshire, are disappointed at Hussain's lack of loyalty towards the county who gave him his big break into first-class cricket.
"He feels that is the right route to get into the England team," added Richardson. "But we don't agree.
"We think the proof of the pudding will be if he gets selected for an England A tour. And, if he does, it'll be on the back of his performances for Gloucestershire.
"It's very disappointing as you've got this guy who came to us last year from nowhere.
"He was scouted out and found by Jack Russell and Stuart Barnes, had one first-class wicket last year and bursts on the scene to be the leading wicket-taker in the country."
Hussain told the county's website: "I want to thank Gloucestershire for the chance they offered me to play and develop my game at first class level.
"I feel I need to challenge myself by playing Division One cricket.
"I believe it will enable me to gain the experience and recognition necessary to be considered to play for England in the future."
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