Smoking rates in England are falling by up to 1% a year and the trend is expected to continue as long as steps to combat tobacco use remain in place, an expert said.
An ongoing survey in England is expected to show "solid" evidence by the end of this year that less than a fifth of the adult population smoke.
Government figures from the General Lifestyle Survey (GLS) have already indicated a UK-wide fall from 39% in 1980 to 21% in 2011.
According to the GLS, smoking prevalence dipped to 19% in England while remaining at 24% in Scotland and Wales.
Professor Robert West, from University College London, who co-heads the Smoking Toolkit Study, said 20% was a psychologically significant marker.
He said: "2013 is going to be, almost without doubt, the first year for a hundred years where we're solidly below 20% smoking prevalence in England. It's going to be a big year. We are making progress. It's slow, and we'd like it to be quicker, but things are going in the right direction."
The Smoking Toolkit Study tracks smoking habits in adults over the age of 16 every month and publishes the results online. Researchers conduct household surveys to collect information from around 1,800 people.
Currently, it shows that average smoking prevalence across England is 19.1%, with a marked difference between populations at the top and the bottom of the socio-economic scale.
Among those in the upper and middle A, B and C1 social brackets, only around 13% smoke. But tobacco is part of the lives of more than a quarter of people classified as C2, D and E. Tobacco use is recognised as the single biggest cause of preventable disease and early death in the UK, claiming more than 102,000 lives a year.
Deborah Arnott, chief executive of Action on Smoking and Health, said: "It is very encouraging that smoking rates are falling; this is the result of having had a comprehensive tobacco control strategy for over a decade. However, the lesson of the past is that new measures such as standardised tobacco packaging need to be implemented otherwise smoking rates are likely to stop declining and may even start to rise again."