SEOUL, South Korea — With his approval ratings poisoned by mass anxiety about American beef, President Lee of South Korea backed away yesterday from a wildly unpopular agreement to resume imports of beef from America. Mr. Lee had personally approved the deal less than two months ago.

Mr. Lee’s change of heart follows weeks of demonstrations by tens of thousands of South Koreans, many of them mothers with children in hand, who are alarmed and angered that his government would expose their families to the purported risks of mad cow disease.

“We have lost the public’s confidence over this matter,” Mr. Lee told his cabinet on yesterday, according to a spokesman.

Plunging poll numbers apparently also played a role in Mr. Lee’s decision to reverse course. Just 100 days in office, his approval ratings have sunk below 20%, a historic low so early in a South Korean president’s term.

His government yesterday asked America to refrain from shipping beef from animals that were more than 30 months old at time of slaughter, which many people here believe raises the risk of mad cow infection to an unacceptable level.

Until the American government complies, it appears that all beef imports will remain on hold.

“It is natural not to bring in meat from cattle 30 months of age and older as long as the people do not want it,” Mr. Lee said, although he had agreed on April 18 to import American beef regardless of age.