(REUTERS) -- Fifty years after top doctors issued their first stark warning about the dangers of cigarettes, more than 20 percent of Britons still smoke -- and unless they quit, half of them will die from the habit, health experts on Tuesday.

Smoking has killed more than six million people in Britain since 1962, when the Royal College of Physicians (RCP) published a landmark report about the health risks of tobacco.

In a new report to mark the anniversary, the RCP said smoking rates have fallen substantially in Britain the past 50 years, and at least 360,000 deaths from smoking have been prevented as people have accepted health advice and quit.

Yet smoking is still the biggest avoidable killer in the UK, said the head of the RCP's tobacco advisory group John Britton, and some 10 million people are still addicted.

Smokers smoke because of an addiction to nicotine that is usually established before adulthood. There is so much more that can and should be done to prevent the death, disease and human misery that smoking causes, he said in a statement.

Unless they quit, half of all smokers will die from their habit, which the RCP said equates to a loss of one hundred million years of life in Britain.

The World Health Organization (WHO) describes smoking as one of the biggest public health threats the world has ever faced. It causes lung cancer, which is often fatal, and other chronic respiratory diseases and is also a major risk factor for cardiovascular diseases, the world's number one killers.

The United Nations health body predicts that smoking could be killing 8 million people every year by 2030 if governments don't take more action to help people quit.

The RCP's 1962 report on smoking and health was the first time senior doctors in Britain had drawn a causal link between smoking and lung cancer, as well as a host of other illnesses.

Its conclusion said cigarette smoking is a cause of lung cancer and bronchitis, and probably contributes to the development of coronary heart disease and various other less common diseases and the number of deaths caused by diseases associated with smoking is large.

Two years later in the United States, the Surgeon General's office issued its 1964 report also warning for the first time that cigarette smoking is causally related to lung cancer and that the risk of developing lung cancer increases with duration of smoking and the number of cigarettes smoked per day.

In Britain, there have been several policy changes aimed at reducing the burden of tobacco-related death and disease, including a ban on smoking in public places, a ban on tobacco advertising and sponsorship, cigarette price rises, a growth of smoking cessation services and restrictions on the sale of cigarettes to children.

The RCP said while these changes have made a difference, there was much more to be done.

Britton called for higher tobacco taxes, noting that although cigarettes are heavily taxed, they are still 50 percent more affordable now in Britain than they were in 1965.

He said the government should also extend smoke-free policies to parks and other public areas, and provide legal protection for children against smoke in cars and houses. He also called for plain cigarette packaging to remove branding.

Australia is seeking to become the first nation to introduce plain packaging on tobacco products by the end of 2012. The packs will show graphic health warnings and banish attractive colors and logos, which particularly attract young people.

Three of the world's four largest tobacco firms, Philip Morris International, British American Tobacco and Imperial Tobacco are fighting that move in Australia's High Court.