The gift is the largest total sum to be given to charity and comes just two months after the couple donated $100m to child immunisation in developing countries.

The money from the chairman of computer giant Microsoft includes $2.23bn to the William H Gates Foundation, named after and administered by Mr Gates' father, and $1.11bn to the Gates Learning Foundation.

Internet access

The William H Gates Foundation provides grants for health, population and education projects worldwide, and various causes in the Seattle area, where Microsoft is based.

Microsoft holdings are worth an estimated $82bn

The Gates Learning Foundation, formerly known as the Gates Library Foundation, is dedicated to educating children in deprived areas in Internet access and computer training. The donation will help take its mission overseas.

The donations will put the William H Gates Foundation in the US top 10 foundations in assets and the Gates Learning Foundation in the top 30.

Last year, the two Gates foundations gave away more than $150m.

Mr Gates' Microsoft holdings are worth more than $82bn, though profits depend on the vagaries of the stock market as he owns 515m shares in Microsoft.

Philanthropic tycoons

News of the contributions was first broken by Fortune magazine's Website. Fortune is owned by Time Warner, the parent company of CNN.

In September 1997, CNN founder Ted Turner made what had been acknowledged as the largest private charitable donation with a $1bn gift to the United Nations over 10 years.

The gift comes as Microsoft is embroiled in a complex court case over its alleged attempts to monopolise parts of the software industry.