Health Secretary Alan Johnson has welcomed the ban on smoking in public places in England as a big step towards a healthier population.
England is the final part of the UK to outlaw the habit, after Wales and Northern Ireland in April and Scotland last March. The Republic of Ireland made the move three years ago.
Virtually all enclosed public places are now smokefree including offices, warehouses, factories, pubs, cafes, railway stations, working vehicles and leisure centres.
Mr Johnson said: "A smokefree country will improve the health of thousands of people, reduce the temptation to smoke and encourage smokers to quit.
"Support for a smokefree England is strong, with almost 80% of the public backing the legislation.
"But we are not stopping here. Hot on the heels of the smokefree legislation, on 1st October 2007, we will be raising the legal minimum age for buying tobacco from 16 to 18 years old."
Owners and managers of pubs, clubs and cafes are legally bound to enforce the ban and face fines of up to £2,500 if they fail to do so.
Anyone caught smoking illegally will be given a fixed-penalty notice of £50 - reduced to £30 if paid in 15 days - or fined up to £200 if they are prosecuted and convicted by a court.
Professor Sir Richard Peto, one of the world's top experts on smoking, said: "Half of all smokers are going to be killed by tobacco. If a million people stop smoking who wouldn't otherwise have done so, then maybe you'll prevent half a million deaths."