The left-hander had long suffered from a stress-related illness and had to pull out of England's 2006 tours to India and Australia.
After playing 76 Tests and hitting 5,825 runs at an average of 43.79, the left-hander called time on his international career in March 2008 due to health problems.
"Regrettably it has not worked out for him regarding his personal well-being and he has travelled back to England to rejoin his family," Brian Rose, Somerset's director of cricket said in a statement. "The Club and all his team mates have fully appreciated this effort and personal commitment and look forward to further success on his behalf," he added.
Even though Somerset has made it to the League stage of the Champions League, Trescothick has not been in best of his form, managing just 17 runs in his two outings.
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