The Uxbridge & South Ruislip candidate won in 2017 by a margin of 5,034 votes.
This is the smallest since 1924, when Labour leader Ramsay MacDonald defended a 3,512 majority in Aberavon in south Wales.
A swing of just over five per cent in west London would be enough for Labour to take the seat from the Conservatives and leave Mr Johnson without a constituency.
This kind of swing, if it was repeated across the country, would see Labour on course to be the largest party in a hung parliament.
Mr Johnson has been the MP for Uxbridge & South Ruislip since 2015.
It currently ranks at number 54 on a list of Conservative seats most vulnerable to Labour.
In 2017 there was a swing in the seat from the Tories to Labour of 6.5 per cent.
A similar swing in 2019 would see Mr Johnson defeated.
He is facing a total of 11 other candidates in this year's election.
Labour, the Liberal Democrats, the Greens and Ukip are all standing, along with the Monster Raving Loony Party, three Independents and three individuals with no party description.
Other PMs who faced voters with slim majorities include Stanley Baldwin in 1923 (5,444), Margaret Thatcher in 1983 (7,878) and Alec Douglas-Home in 1964 (9,328).