Mr Lee, who turns 85 tomorrow, was warded for a day for atrial flutter, which is not uncommon among people of his age.

Doctors have pronounced his condition to be stable, said the Prime Minister's Office.

He has been advised to resume his official schedule gradually.

Despite being hospitalised, the Minister Mentor kept his appointment with an international philanthropy forum at the Arts House on Saturday night, addressing the participants for five minutes through a video link.

Mr Lee, who had two operations in 1996 to clear a blocked heart artery, is known to exercise regularly.

In an e-mail interview with The Straits Times last week, he said he cycles and swims daily and eats moderately to keep in shape.