MPs will tonight decide whether to impose a total ban on smoking in public places in England, with supporters of the measure expected to win the day by a small majority.
A total ban will be implemented in Scotland in April, with Wales and Northern Ireland due to follow suit a year later.
When Labour first proposed a smoking ban as part of its 2005 election manifesto, a total ban was not on the agenda.
Then, health was the remit of John Reid - a reformed smoker who wanted smoking banned in all public places except pubs and clubs not serving prepared food.
Since then, the cabinet has been under pressure to opt for a full ban amid vigorous lobbying by anti-smoking campaigners.
The issue has divided the cabinet, with the current health secretary, Patricia Hewitt, keen on a blanket ban but Mr Reid arguing that would not be what was promised.
Repercussions were felt in the civil service, with the chief medical officer, Sir Liam Donaldson, considering resignation over the government's refusal to entertain a full ban.
Although Tony Blair backed Mr Reid's stance, he later relented to allow MPs a free vote on the issue.
Earlier this year, 10 members of the 11-strong health select committee backed a rebel motion tabled by the chairman, Kevin Barron, calling for a full ban in order to protect workers from second-hand smoke.
Mr Barron's move worked - soon afterwards, the government announced two amendments that could see a full smoking ban in place by 2007.
The first vote on the issue, expected to take place at around 9pm, will decide whether private members' clubs should be exempt from a ban.
An amendment tabled by the public health minister, Caroline Flint, will determine whether they should be kept on a list of premises escaping the ban under discretionary "general powers of exemption" in the health bill.
Those powers mean the government would be able to ensure that people in - for example - care homes, psychiatric units and prisons would not be subject to a ban.
This vote will feed into the main clause of the bill, on whether or not a smoking ban should be imposed on all licensed premises or just places serving prepared food.
Ms Hewitt is thought to be leaning towards allowing private members' clubs to stay on the list, while Ms Flint is expected to vote for a full ban.
A full ban has the support of the home secretary, Charles Clarke, the Labour party chairman, Ian McCartney, the communities and local government minister, David Miliband, and the Northern Ireland secretary, Peter Hain.
Meanwhile, most of the 63 Liberal Democrat MPS are expected to vote for a total ban - part of the party's general election manifesto. A small number of Lib Dems are expected to join Mr Reid's camp.
Few Conservatives are expected to back a full ban, although around 20, led by Sir George Young, are in favour. The final outcome of the votes will depend on how many MPs turn out to have their say.
It is more than three years since a private members' bill on a smoking ban was first put forward, paving the way for the government's health bill.
The proposal received widespread support from organisations such as the British Medical Association and the Trade Union Congress.
Anti-smoking campaigners hope their lobbying for a full ban will pay off. A total ban comes into effect in Scotland in April, while the Welsh Assembly is waiting for the health bill to receive royal assent so it can implement a full smoking ban next year. A full ban will also come into force in Northern Ireland in 2007.
The bill raises "the West Lothian question" - MPs from UK countries with devolved governments can vote on issues pertaining only to England, but English MPs have no say on the smoking bans in those countries.
