It follows an European Union directive which said they should cover 30% of the front of cigarette and 40% of the back.
Health warnings on the back of packs
General warnings such as "Smoking kills", "Smoking can kill" or "Smoking seriously harms you and others around you" will appear on the front of packets.
One of 14, more specific, warnings will appear on the back (see box). These will be rotated so that they will all appear at regular intervals
The warnings will also be highlighted by thick black borders.
Warnings block
Tobacco companies recently lost their fight against the tough new rules, which also mean the terms "light" and "mild" are banned.
The introduction of the warnings comes as it emerges former Tory Prime Minister Edward Heath curbed attempts to clamp down on smoking in the early 1970s.
New Scientist magazine reports the then social services secretary Sir Keith Joseph was proposing a bill which would have forced cigarette adverts to carry health warnings and give local authorities the powers to ban smoking in public places.
But the idea was quickly watered down. In 1971 tobacco companies agreed to carry health warnings on packets as part of a voluntary code, but no legislation was introduced.
Reminder
Amanda Sandford, research manager for the charity Action on Smoking and Health welcomed the new EU health warnings.
"Very soon there will be no escaping the stark facts about smoking.
"Every time smokers reach for a cigarette they will be reminded of the health consequences of lighting up.
"Although everyone knows smoking is harmful, it's easy to forget just how dangerous it is.
"These new warnings will be a constant reminder that smoking causes fatal diseases."
She said the harsh warnings may help spur on those smokers who want to mark the New Year by quitting.
Ms Sandford advised people who did want to stop smoking to ask their GPs or pharmacists for advice.