Under what was being described yesterday as a tentative agreement between the US, the UN and congressional leaders in Washington, Mr Turner will make up the $35m (£24m) difference between what Washington owes and what the Republican-dominated Congress is willing to pay.

"We have the contours of a deal," a spokesman for the US mission to the UN said in New York yesterday. "Details need to be finalised."

If approved, the agreement will cut the American share of the UN's running costs from 25% to 22%, as well as reducing Washington's share of the UN peacekeeping budget from 31% to less than 27%.

The cuts will pave the way for Congress to release enough money to pay the bulk of the US debts of $1.8bn to an international body seen by many Republicans as spendthrift at best and as an anti-American international conspiracy at worst.

Mr Turner, the founder of CNN, is a passionate supporter of the UN and has already pledged to donate $1bn of his personal fortune to the organisation over 10 years. Until now, however, he has stipulated that his money must be spent on international programmes rather than on paying the US government's dues.

But Mr Turner had a change of heart five weeks ago at a board meeting of his United Nations Foundation, the body that manages his UN charitable work. Mr Turner told America's permanent representative to the UN, Richard Holbrooke, that he would make up the difference to enable the US to balance its accounts with the UN for the year 2001.

Negotiations had stalled because most UN member states had settled their own national budgets for 2001 and were unwilling to pay more to cover the American shortfall.

The UN is not allowed to accept money directly from individuals to pay for a country's dues, so Mr Turner's money would be paid to the state department, enabling the US fees to be settled.

"Ted's gesture is extraordinary and visionary," Mr Holbrooke said yesterday. "I hope it proves to be the key that unlocks this extraordinarily complex problem."