Smoking in cars that are carrying children has already been banned in some states of Canada, Australia and the US and the whole of South Africa.
Canada's Cancer Advisory Coalition claims the states' bans on smoking in cars have reduced children's exposure to second-hand smoke by 33%.
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Tory Eurosceptic Jacob Rees-Mogg played down the significance of the deal struck by the Government to win over pro-EU rebels.
The Tanaiste issued the warning to the UK ahead of a visit to Dublin by European Commission President Jean-Claude Juncker.
Around 9,500 people gathered at the neolithic World Heritage Site to attend the overnight celebration of the longest day of the year.