Keep Singapore Clean campaigns in the early decades after independence led to spotless streets, but standards have dipped in recent years.
Mr Liak Teng Lit, chairman of the Public Hygiene Council, which leads the Keep Singapore Clean Movement, told The Sunday Times: "The improvement was very great in that, starting from the late 1970s and right into the 1980s and early 1990s, Singapore was truly very clean. Very few people littered."
But this has changed in more recent years, he said, with the sharpest decline occurring in the past two to three years.
He feels there used to be a greater emphasis on keeping the place clean. Newspapers ran pictures of leaders sweeping the floor and litterbugs cleaning public places after being slapped with Corrective Work Orders.
These days, he said, "we are reluctant to shame people who litter".
Some also take cleanliness for granted and see it as the job of cleaners.
The result is that a critical mass of litterbugs has formed. Mr Liak estimates this sizeable minority at about three in 10 people.
So it is common to see rubbish strewn outside MRT stations, convenience stores or fast-food eateries.
Things have reached a worrying level, and there's a need to change social norms, he said.
It's not enough for the National Environment Agency to step up enforcement. He feels people should speak up when they see others littering.
"The average Singaporean should stand up and say that this is not acceptable," he said.