Lee, 91, has been hospitalised at the Singapore General Hospital since February 5 for severe pneumonia.
“Mr Lee Kuan Yew’s condition has worsened due to an infection. He is on antibiotics. The doctors are closely monitoring his condition,” said a statement from the office of his son, Prime Minister Lee Hsien Loong.
The senior Lee is widely credited with transforming Singapore from an economic backwater to one of Asia’s wealthiest economies in just over three decades.
He served as prime minister from 1959, when Singapore gained self-rule from colonial ruler Britain, until he stepped down in 1990 in favour of his deputy Goh Chok Tong, who in turn handed power to Lee Hsien Loong in 2004.
The People’s Action Party, which was co-founded by the elder Lee, has been returned to power in every election since 1959 and currently holds 80 of the 87 seats in parliament.
In a book published in 2013, the Asian statesman said he feels weaker by the day and wants a quick death.
The longtime fitness buff has visibly slowed since his wife of 63 years Kwa Geok Choo died in 2010.