Smokers across England have sparked up at work and in the pub for the last time as the ban on smoking in enclosed public places begins.
People caught illegally smoking could be fined up to £200
The new law, which came into effect at 0600 BST, is intended to cut deaths from second-hand smoke.
Scotland, Wales and Northern Ireland already have similar bans in place.
Many venues held farewell events for the final night of smoking on Saturday, while local authorities are preparing to enforce the ban.
Doctors estimate second-hand smoke kills more than 600 people a year.
The government also hopes it will help smokers to quit, and discourage children from taking up the habit.
The new Health Secretary, Alan Johnson, welcomed the ban saying that tackling the causes of illnesses saved lives.
"A smoke-free country will improve the health of thousands of people, reduce the temptation to smoke and encourage smokers to quit," he added.
England's chief medical officer Sir Liam Donaldson said the ban was a "momentous move" and would prevent the deaths of both smokers and non-smokers.
"We are removing from the air at a stroke 50 cancer causing chemicals, and that's bound to be good news for the exposure to risk," he said.
From Sunday anyone lighting up illegally could be fined £50 - reduced to £30 if it is paid within 15 days.
The figure rises to £200 if an individual is prosecuted and convicted by a court.
Businesses failing to comply with the ban could be hit with fines of up to £2,500.
The ban has prompted protests by smokers and those concerned about what they see as the "nanny state".
A legal challenge to the ban has been launched at the High Court by Freedom2Choose, which says the change in the law contravenes the European Convention on Human Rights.
Smokers from the group defied the ban The Dog Inn in Ewys Harold, near Hereford.
Landlord Tony Blows said he was prepared to go to court.
Mr Blows said: "I'm doing it for the simple reason that this is my home. My
wife and I work 200 hours a week in this pub. It's private property and there's no way they can stop us doing it.
"Pubs have been smoking for goodness knows how long and you just can't do that. It's been brought in on the back of a pack of lies."
Decline in sales
Others are worried that the ban will mean the demise of the traditional pub and other social haunts such as middle-eastern style shisha cafes.
Market researchers Nielsen estimate beer sales in England and Wales could drop by 200 million pints each year because of the ban.
However, a survey by the Campaign for Real Ale suggested England's 6.2 million regular drinkers are likely to go out to pubs and bars more often after the ban.
Its study also found that 840,000 people who currently do not go to the pub said they would do so after smoking was made illegal.
Mark Hastings, communications director of the British Beer and Pub Association, said that although the ban may lead to a small decline in beer sales, pubs would also see an increase in the sale of food.
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