Fund management company City of London Investment has written to Blackrock to complain over the share price performance of one of Blackrock's funds, the latest example of a fund coming under attack from activist investors.

City of London Investment's move comes after Alliance Trust fought off a resolution from Laxey Partners in May to set up an automatic buyback policy aimed at boosting its share price.

City of London Investment Group said it written to the directors of Blackrock World Mining Trust to express disappointment with the fund's share price, despite its strong returns.

BRWM's investment returns over the three years to the end of October 2011 have exceeded 150 percent. This is very satisfactory compared to the benchmark HSBC Global Mining Index return over the same period of 128 percent and we congratulate the investment manager, it said in its letter.

However, we are disappointed that this good investment record is not more fully reflected in the share price, which remains at an unreasonably large discount to NAV (net asset value). This is clearly an area where the board needs to be more assertive, it added.

We have written to you previously on this issue in April 2010 and in June 2011, and we consider that the time for action is long overdue. We have decided to publish this latest letter in order to encourage other shareholders to join us in requesting that the board adopt best practice in respect of discount control, it wrote.

Blackrock officials in London had no immediate comment on the matter.

Shares in Blackrock World Mining were down 1.8 percent at 606.90 pence in early afternoon trade. The stock has fallen around 25 percent over the last year.

The Blackrock World Mining Trust is run by Evy Hambro, a member of Britain's historic Hambro banking family.

Companies in which the fund has invested include miners Rio Tinto and BHP Billiton .

(Reporting by Sudip Kar-Gupta and Sinead Cruise; Editing by Will Waterman and David Holmes)