A campaigning anti-smoking charity is warning UK bosses that they now face legal action if they fail to protect their workers from tobacco smoke.
The government does not currently support a ban
Action on Smoking and Health says that all employers should now be well aware of the health threat posed by smoke.
The charity is campaigning for bars, clubs and restaurants to be made smoke-free zones to protect staff.
The hospitality industry is opposed to a complete ban, saying it is already improving air quality in pubs and bars.
In a letter to the trade, ASH warns that a so-called "date of guilty knowledge" - at which point no employer should be unaware of the potential health damage caused by passive smoking - has long past.
This fact, it says, renders any employer not taking steps to protect staff vulnerable to court action under the Health and Safety at Work Act.
It has teamed up with a firm of personal injury lawyers and plans to use the letters as evidence in any future court action.
Last year representatives from all 13 Royal Colleges of Medicine called for a ban on smoking in public places, saying the move could save 150,000 lives.
It is estimated in some surveys that eight million non-smokers were exposed to smoke in their workplaces.
Deborah Arnott, ASH director, said: "The time is long past when employers should have known that second-hand smoke is bad for their staff, and bad for the general public.
"To make quite sure that they understand this crucial point - and the rising threat of legal actions that they now face - we have sent all the UK hospitality trade's leading employers a formal registered letter.
"This spells out the science on second-hand smoke, and the law as we understand it."
At Thompsons, the legal firm involved in the initiative, John Hall said: "Bosses are no more entitled to allow smoke in the workplace as they are to allow asbestos or coal dust.
"They need to give the order to stub out or they will face the growing threat of legal action."
ASH want restaurants to be smoke-free zones
However, a total ban on smoking in pubs and restaurants may not enjoy widespread support.
According to a survey by pollsters BMRB published last year, only 17% of those questioned thought there should be a total ban on smoking in pubs, clubs and bars.
Although 75% believe further improvements are necessary, 61% agreed that pubs, clubs and bars are noticeably less smoky than they were.