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Green has run the Global fund since 1984 and the UK fund since 1990, and will step down from both at the end of the year.
Since August 1990 his UK fund has returned an outstanding 2,277% versus a 906% rise in the FTSE All-Share.
He also performed exceptionally on the Global fund, returning 784% since December 1993, versus a 541% rise in the FTSE World index.
Although he currently does not hold a Citywire rating, his strong risk-adjusted performance over the long term means he has been rated for prolonged periods over the last 15 years. This included a spell where he held a rating every month for five consecutive years from September 2002.
From the start of next year Chris Morrison and Adrian Gosden will co-manage the GAM UK Diversified fund.
The Global fund will pass to Ali Miremadi, who joined in 2016 from THS Partners, a long-standing sub-adviser to the GAM Star Worldwide Equity fund.
Green will continue to manage a few select mandates and will also provide advice and support to GAM equity teams in the new role of director of global equities strategy.
'I was fortunate to join GAM when it was first founded by Gilbert de Botton in 1983, and I look back fondly on those fascinating 34 years with the company,' Green said.
'Gilbert’s entrepreneurial spirit and innovative approach to meeting clients’ needs through active management are still at the core of GAM’s DNA.
'True to these values, Adrian, Chris and Ali are passionate about delivering meaningful returns for clients, and I wish them every success in the future. I look forward to working with GAM in my new advisory capacity.'