Investors who put pounds 100 into the fund in 1989 would have made a profit: their investment would be worth pounds 240.84 today. But the same amount invested in the FT-SE All Share index would have reaped pounds 412.95.
Simon Males, business development manager at Friends, Ivory and Sime, the investment house which runs the Stewardship fund, argues that the findings are unfair because the FT-SE All Share is not the best benchmark to use to measure performance.
"One of the key factors that differentiates the fund from other UK equity funds is that the Stewardship is constrained by its ethical criteria," he says. "There is a 50:50 exposure between large and small-cap companies, while the FT-SE All Share is very differently weighted, focusing primarily on larger companies."
Instead, Friends Provident uses a composite index of the FT-SE All Share and the Hoare Govett Smaller Companies index to measure the performance of the fund.
But the findings are bad news for fund managers who have worked hard to make environmentally friendly investments credible. Launched in 1984, the Stewardship fund was the UK's first ethical fund. Although it got off to a slow start, with people dismissing ethical investing as cranky and suitable only for the "brown rice and sandals" brigade, pounds 3bn is now invested across 40 UK ethical funds.
The question is whether investors will be so keen to follow their conscience once they realise that it is not as good for the pocket.
But the Stewardship fund was not the worst performer of the decade. That dubious honour goes to the MGM Special Situations Growth fund. This underperformed the FT-SE All Share by 63 per cent over 10 years.