Smoking should be outlawed in public parks, the chief medical officer said last night.

Sally Davies claimed a ban was needed to stop children being set a bad example.

Campaigners immediately denounced the idea as an attack on personal liberties.

They said it could be the start of a slippery slope leading to a total ban – even in homes.

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Slippery slope? Chief medical officer Sally Davies (pictured) has suggested smoking should be banned in parks

The plans are in a report commissioned by Boris Johnson and written by Lord Darzi, a surgeon and former Labour minister.

The peer calls for Mr Johnson to use his powers as mayor to ensure 20,000 acres of parks in London go smoke-free. The ban would even extend to Trafalgar Square and possibly nationwide because Lord Darzi said it would be a blueprint for the whole of England.

Dame Sally said it was dangerous for children to see adults smoking in parks because they might decide to copy them. ‘We all know smoking is bad for health,’ she said. ‘So I welcome any measures to reduce both active smoking and its role-modelling in front of children.’

But Stephanie Lis, of the Institute for Economic Affairs, said: ‘This is an outrageous attack on personal freedom – a slippery slope on the path to banning smoking altogether.

Park: People lighting up in public encourages children, the report said (picture posed by model)

‘Politicians must avoid this heavy-handed interference in the lives of ordinary people.’

And Simon Clark, of the smokers’ group Forest, said: ‘A ban on smoking in open air parks would be outrageous.

'There’s no health risk to anyone other than the smoker. If you don’t like the smell, walk away.

‘The next thing will be a ban on smoking in our own gardens in case a whiff of smoke travels over the fence.

‘The anti-smoking bandwagon is becoming farcical. It’s sad to see the Mayor of London encouraging such nonsense.’

Mr Johnson said the proposal should spark public debate. ‘One of the glories of London is that we are generally pretty laissez-faire about how people live their lives – provided they do not break the law and provided they do no harm to others,’ he said.

‘If we were to consider a ban on smoking in parks, we would need pretty clear evidence that this would have direct health benefits – in other words, that it would actually save lives. It is time for London to have that debate.’

The Government’s public health quango backed the idea of smoke-free parks.

Rosanna O’Connor, who heads the alcohol, drugs and tobacco division of Public Health England, said: ‘Parks and outdoor spaces should be healthy environments for people of all ages. Smoke-free outdoor spaces will help de-normalise smoking in our society by reducing the number of smoking role models for children.’

Professor Robert West, director of tobacco studies at University College London, said: ‘I would expect it to save lives by reducing reminders of smoking in those who are trying to quit and providing more incentive to stop.

‘I hope most smokers would see this as a positive step.’

Optimistic: The report for Boris Johnson aims to turn London into a healthier capital (picture posed by models)

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Lord Darzi’s report, called Better Health for London, calls for a range of measures to improve the health of people in the capital – leading to a ‘healthier, slimmer, fitter city’. It called for mandatory traffic-light labelling on restaurant menus, public transport discounts for those who walk for a substantial part of their journey to work and restrictions on junk food outlets near schools.

Councils with a particular alcohol problem should also use their licensing powers to introduce a minimum unit price of 50p. This final recommendation would be seen as a challenge from the London Mayor to David Cameron. The Government has so far declined to introduce minimum pricing.

Lord Darzi said Mr Johnson should use his powers over Trafalgar Square and Parliament Square to ensure they go smoke free.

He should also use his influence over the Royal Parks, the board of which he appoints. This would take in Hyde Park, Richmond Park, St James’s Park, Green Park, Regent’s Park, Kensington Gardens and Greenwich Park. Town halls would be encouraged to do the same in the parks they control.

It is hoped cities and towns up and down the country would follow suit.

‘The ideas and proposals in this report have been developed for London,’ said Lord Darzi. ‘Yet they could just as easily apply to other big cities in the UK.

'Hundreds of children take up smoking every week – two classrooms-full a day in London. Once they start they continue, since cigarettes are more powerfully addictive than narcotics.

'We know smoking is bad for health,' said Dame Sally, 'So I welcome any measures to reduce role modelling'

‘It’s little surprise that in places where more adults smoke, more children begin smoking too. London should lead the way for Britain, and the Mayor should lead the way for London by acting to make our public spaces smoke free.’

Asked how smoke-free parks would be policed, Lord Darzi said 98 per cent of people abided by the ban on smoking in pubs and restaurants which was introduced in July 2007.

‘I don’t see the necessity of having wardens walking around the parks,’ he said. ‘The evidence will suggest that our nation does abide with the laws. We just need very clear signs.’

The plan was welcomed by Michael Bloomberg, the former mayor of New York City, where smoking is already banned in all parks.

He said: ‘London’s plan will help prevent children from picking up the habit, discourage adults from smoking, and improve the health of children and families.

‘I look forward to visiting London’s smoke-free parks and squares, and I’m certain that millions of others will as well.’

Deborah Arnott, of the anti-smoking charity ASH, said: ‘London needs to do more to tackle smoking, which is the major cause of preventable premature death in the capital.’

A source close to Health Secretary Jeremy Hunt said he was not considering extending smoking bans to public parks, though London and local authorities could act independently to implement their own measures.