Smith, 31, who was appointed as Middlesex's captain in 2007, missed most of last season with an ankle injury and as the summer progressed there were increasing rumours that all was not well within the dressing-room. In September, the county announced that Shaun Udal would lead the side in 2009, and thereafter it was accepted that Smith had played his last game for the county.
"It's been a real honour to captain Middlesex," Smith said. "I've been very lucky to play first-class cricket for eight years at Kent and four years at Middlesex. I'd like to thank all the players, coaches and supporters who have helped me enjoy the game I love. I wish Shaun Udal and the team every success in the future."
A tall right-hander with a penchant for the drive, Smith won his three Test caps in 2003 on the strength of excellent form for Kent, hammering six hundreds and becoming the first player to pass 1000 runs. But despite a fifty in his maiden innings against South Africa, he managed only 23 in the next four innings and was dropped.
In 2004 there were rumours that he had become alienated from the Kent dressing-room, and it was no surprise when he moved to Middlesex at the end of the summer. After two consistent seasons he made the natural progression to leadership when he was named captain for the 2007 season.
A voracious reader and writer, he picked up a double-first in history despite devoting much of his time at Cambridge University to cricket, and opened for England Under-19s in three Tests against New Zealand in 1996.
In 191 first-class matches he made 12,789 runs at 41.79, including 34 hundreds. He also made 3789 runs in one-day games at 31.31 and 573 at 22.92 in Twenty20.