Boris Johnson's majority of just 5,034 in his parliamentary seat has made Uxbridge and South Ruislip a target for Jeremy Corbyn's Labour Party.
Labour candidate Ali Milani, 25, requires a swing of around five per cent to steal the prime minister's title as representative of the constituency.
If that were to happen on December 12, Mr Johnson would become the first sitting prime minister to lose their seat at a UK general election.
What would that mean for Mr Johnson's role as the head of government?
Tradition dictates that the prime minister should be a member of the House of Commons.
On that basis, then, it is likely that Mr Johnson would cease to be the occupant of No. 10 Downing Street.
Yet it has not been unknown for someone to hold the highest office in government without being an MP.
The last time that happened, however, was when Lord Salisbury was in charge from 1885 until 1902.
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