Up to 40 per cent of pubs could escape the ban if ministers proceed with a law which exempts pubs that do not serve food. The survey by Cancer Research UK and Ash, the anti-smoking charity, found that 29 per cent of pubs at present do not serve food. But this proportion is set to rise by a third to 40 per cent, and up to 50 per cent in the poorest areas, if the law due to come into force in two years' time allows smoking in pubs that serve drink only.

The findings are released on the last day of the consultation on smoke-free workplaces which has provoked widespread criticism of the Government's plans for a partial ban.

Cancer Research UK and Ash said they confirmed fears that a partial ban would widen the health gap between rich and poor.

The survey of 1,252 publicans, landlords and managers across England and Wales was made to gauge the proportion of pubs currently serving food, as well as publicans' plans once smoke-free legislation is introduced. Deborah Arnott, director of Ash, said: "This survey shows that the Government is threatening to undermine the enormous public health benefits of its smoke-free legislation by exempting many pubs and clubs. These will be heavily concentrated in the poorest communities across the country - areas generally represented by Labour MPs. Exemptions would leave many workers at most risk from the damage caused by second-hand smoke. This research should be the final evidence the Government needs to drop the exemptions."

The British Medical Association accused ministers of double standards for launching a campaign today warning of the dangers of passive smoking. Vivienne Nathanson, head of science at the BMA, said: "If the Government is aware of the hazards [of passive smoking], how can it defend only a partial ban on smoking in public places - exposing workers to toxic chemicals just because they are unlucky enough to work in pubs and bars not selling food?"

The British Heart Foundation said: "Protecting only 60 per cent of pub workers from the dangers of second-hand smoke because they happen to serve burgers with their beer is a senseless proposal that must be stubbed out without delay."

The Royal College of Nursing (RCN) has urged the Health Secretary, Patricia Hewitt, to bring in a full ban on smoking in enclosed public places. The RCN said this would save 30 people a day from the fatal effects of passive smoking.