Prime Minister Lee Hsien Loong thanked the late eye surgeon Robert Loh for his "selfless devotion" in a letter to his widow yesterday.

Dr Loh, who died of a stroke on Tuesday, was the first president of the National Council of Social Service (NCSS) and the longest-serving president of Christian charity YMCA. He was 91.

"We are deeply grateful to Dr Loh for his selfless devotion to the community, remarkable leadership and many contributions.

"He strengthened Singapore's social service sector, VWOs and the medical profession, and strived to ensure that no one was left behind as the country progressed," Mr Lee wrote in his letter to Mrs Mary Loh.

Mr Lee listed Dr Loh's many achievements in the letter, including transforming the original Singapore council of service into the NCSS.

"The pioneering work by Dr Loh and his team helped to build a strong foundation for the social service sector," said Mr Lee.

Dr Loh was also YMCA's president from 1970 to 1992, and a pioneer ophthalmologist who "inspired several generations of doctors", he added.

Dr Loh received four National Day Awards, including the Meritorious Service Medal in 2002.

On a more personal note, Mr Lee wrote: "Personally, I was privileged to have known Dr Loh as a friend.

"He was always gentle and modest in his manner, an unassuming but respected elder who wore his achievements and contributions lightly.

"He attended to my children when they were small, for which I will always be grateful," added the Prime Minister.

Minister for Social and Family Development Tan Chuan-Jin also lauded Dr Loh's contributions in a Facebook post yesterday.

"Dr Loh's lifelong contributions to the social service sector have left an indelible mark on the lives of many Singaporeans.

"He will be dearly missed by all of us," Mr Tan added.