In England, fewer than one in five adults now smoke, the smallest fraction in three generations, reports study published in British Medical Journal (BMJ).
In a letter, health researchers Jamie Brown and Robert West of University College London reported on a seven-year-old programme to monitor smoking prevalence in England.
It samples 1,800 people aged over 16 who are randomly chosen each month.
In 2013, 19.3 percent of those questioned said they smoked.
"For the first time in probably 80 years, smoking prevalence in England has fallen below 20 percent," the letter said.
Smoking in England was rare at the start of the 20th century and reached a peak in the early 1960s, when more than 70 percent of men and 40 percent of women smoked.
Brown and West said smashing the 20 percent mark could be an important moment in rolling back tobacco.
The decline in smoking prevalence in England began in the 1970s, they said, with an annual fall of 0.6 percentage points a year on average. The retreat in 2013 was slightly higher at 0.8 percent.
"Much is still to be done," the pair wrote. "However, we hope that breaking the 20-percent barrier will motivate smoking cessation efforts across the country."