But these contain toxins including arsenic and lead.
They get washed out to sea where they present a risk to wildlife.
One study showed that just one cigarette butt per litre of water is highly toxic to fish.
The warning opens a new front in the campaign against marine pollution which in recent years has focused on plastic.
KBT today launches a nationwide campaign, Bin the Butt, to try to stamp out cigarette litter.
Chief executive Allison Ogden-Newton said: “Following Sir David Attenborough’s rallying cry to reduce plastic waste on Blue Planet II, we wanted to show how simple everyday behaviour can affect the environment. While flicking a cigarette down the drain may not seem harmful, we need to ensure smokers understand that this has a direct and often drastic impact on wildlife.”
KBT found that just 53 per cent of smokers knew that cigarette butts get washed into the sea if they are dropped down the drain.
A third of people who smoke every day wrongly thought that they are filtered out during water treatment.
Four in 10 smokers, equivalent to four million Britons, admitted to throwing fag ends down the drain in the past month.
Bizarrely, one in 10 did not even think of them as litter.
Ms Ogden-Newton added: “Our research showed that more than a fifth of Britons thought putting a cigarette down the drain was acceptable, which rose to over half among smokers who smoke every day.
“We need to challenge this view, and get the message to smokers that this is still littering.”
Among smokers unaware of the threat to marine life was comedian Johnny Vegas who in May was fined £150 by Doncaster Council after dropping a cigarette end down the drain.
At the time Vegas said he thought he had behaved responsibly. He wrote on social media: “Dear MyDoncaster whilst my love for the good folk here will never diminish your £150 fine for responsibly placing a cig in a grid has now placed us at odds.”
But the council tweeted back: “At the risk of becoming the butt of the joke, drains aren’t appropriate disposal areas for cigarettes. These eventually end up in our water courses and damage the environment.”