Dr Les King, a chemist, quit the Advisory Council on the Misuse of Drugs Advisory Council on the Misuse of Drugs and said that the Home Secretary had denied the chairman’s right to free speech when he sacked him.

He was followed by Marion Walker, a pharmacist, who is clinical director with the substance misuse service at the Berkshire Healthcare NHS Foundation Trust.

Professor David Nutt, chairman of the council, was dismissed after saying that cannabis was less harmful than alcohol or nicotine and had been reclassified for political reasons.

Dr King, who became a full member of the council last year, said that the Government’s attitude to the panel had been shifting in recent years and Home Secretaries now had a “pre-defined political agenda” when they asked for its expert advice.

“It’s being asked to rubber stamp a pre-determined position,” he said. “If sufficient members do resign, the committee will no longer be able to operate.”

Dr King said he believed that the panel needed to become “free from government interference” in the same way as the National Institute for Health and Clinical Excellence (NICE), the organisation that advises on medicines and clinical practice.

“I don’t see why drugs can’t be done the same. It can be totally depoliticised. It’s all about harm. It’s a scientific issue,” he said.

It was launched as a wonder drug. A tablet to take the pain out of kicking the killer weed.

Since it appeared on the market two years ago in the UK, almost 400,000 prescriptions for Champix have been written.

Across the world, that figure is currently sitting at nine million.

Champix, or varenicline, to give it its clinical name, was first licensed here in December 2006.

It mimics the effects of nicotine on the body so it both reduces the urge to smoke and relieves withdrawal symptoms.

In clinical trials, it proved more effective than alternative remedies at getting people to quit.

For its makers - Pfizer - Champix has been a huge success.

In its first full year on the market, the drug brought in a staggering $883m for the company.

But about a year ago, I became aware of stories emerging in the media in which people who had taken Champix were said to have suffered severe depression.

Tobacco use could kill a billion people this century unless governments act now to reduce smoking, the UN said yesterday.

In a strongly-worded report the World Health Organisation, the UN's public health arm, said no country was doing all it could to curb tobacco use, which is set to kill 500 million of the world's current population and a further 500 million over in the course of the century.

It called on all countries to adopt six measures to cut the numbers taking up smoking, especially young people and women, and to help smokers quit. Increasing taxes to 75% or more of the pack price would be the single most effective strategy, the WHO said. Raising taxes could also provide funds to counter tobacco industry marketing tactics.

The WHO's six measures are: monitoring tobacco use and prevention policies, protecting people through smoking bans, offering help to quit, warning about the dangers of tobacco, enforcing bans on promotion, marketing and sponsorship, and raising taxes.

"While efforts to combat tobacco are gaining momentum, virtually every country needs to do more," said Dr Margaret Chan, director general of the WHO. Only 5% of people live in countries that fully implement any one of the six measures, the WHO said.

The big target for the tobacco industry is the developing world, according to the report, where the use of cigarettes and other forms of tobacco is growing fast. "As many as 100 million Chinese men currently under age 30 will die from tobacco use. In India about a quarter of deaths among middle-aged men are caused by smoking," the report said.