Farrow visited Nigeria last week

The 55-year-old actress contracted polio when she was nine, but recovered. Her 12-year-old adopted son, Thaddeus, became infected in an orphanage in Calcutta, India, and is paralysed from the waist down.

"I live with the effects of polio every single day," she told a New York press conference. "So I feel doubly motivated to see the end of polio."

Last year, Nigeria reported 500 polio cases, roughly a quarter of the global total, according to Unicef figures.

The UN campaign is one of the largest public health initiatives ever, and is targeting parts of Africa and South Asia where the disease is still active.

Unicef's deputy executive director, Kul Chandra Gautam, said the reappearance of polio in the Dominican Republic and Haiti a few months ago showed it could still spread to areas which had been declared polio-free a decade ago.

"Until polio is eradicated everywhere, it will not be eradicated anywhere," he said.

But the UN is $450m (�310m) short of the $1bn (�690m) it neets to complete the programme by 2005, even though officials say eradication of the disease will actually save governments $1.5bn each year in immunisation costs.

In 1998 British singers Ian Dury and Robbie Williams took part in a similar trip to Sri Lanka to take part in an immunisation programme.

Dury, who died of cancer in March 2000, also suffered from polio as a child.