The chairman of internet search firm Yahoo, Terry Semel, has quit as chief executive and has been replaced by co-founder Jerry Yang.

In the post since 2001, Mr Semel has been under pressure for some time amid disappointing trading results and the growing dominance of rival Google.

The firm suffered a 16% fall in profits in the first three months of 2007.

Mr Yang helped launch the business, which now has more than 500 million users worldwide, in 1995.