Manassero took control after the morning round to take a 3-up lead, then pulled away from Sam Hutsby of England to win the title, 4&3.
Manassero, winner of the 2007 European Young Masters, was the youngest player to qualify from the Amateur Championship since David Duval in 1991. Today's win has also done his chances no harm towards representing Italy in the European Team Championships later in the season.
At the BMW Italian Open last month Manassero had a respectable 25th finish, the experience of playing the European Tour obviously doing no damage to his chances this week at Liverpool.
Winning the British Amateur was Manassero's fourth title for the season after collecting the Italian Amateur Stroke, the Palla D'Oro Championship (Italy) and Medalist honours at these Championships.
Today's victory makes Manassero eligible for the British Open at Turnberry next month as well as the 2010 Masters.
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