The investment has been made through the Bill and Melinda Gates Foundation, the largest charitable body in the US with an endowment of $24bn (£16.5bn). It has become a significant force in international health issues and contributed $555m to programmes last year alone.

The decision to take stakes in individual firms appears to be a shift in strategy, and for the first time aligns the charity's interests with those of the drugs firms. The foundation had eschewed equity investments and held shares in just two companies - cable firm Cox Communications and Waste Management. Now it has ploughed $76.9m into Merck shares, $37.3m into Pfizer and $29.7m into Johnson & Johnson.

Mr Gates has already built some ties with the drugs industry. Merck chief executive Raymond Gilmartin joined the Microsoft board last year while Mr Gates helped Merck with AIDS programmes in Botswana - much of the foundation's focus has been on improving health in the developing world.

Investment in drugs firms could leave the foundation open to criticism. A representative sits on the board of the Global Alliance for Vaccines and Immunisations which buys vaccines from some of the pharmaceutical firms in which the foundation now holds shares.

A spokesman for the foundation said the investments were independent of the charitable programmes.

Mr Gates' belief in the importance of intellectual property protection for drugs in the developing world as a means of encouraging investment is also controversial.

The foundation was created in 2000 through the merger of the Gates Learning Foundation and the William H Gates Foundation and is chaired by Bill Gates senior.