The former prime minister was writing letters and doing campaign videos for some candidates, including in the so-called "Red Wall" in the North and Midlands.
In one for Alice Hopkin, Tory candidate in Normanton & Hemsworth, West Yorkshire, he warned voters against giving Sir Keir Starmer a "super-majority".
His intervention comes as polls pointed to voters deserting the Conservatives for Reform in significant numbers, which could cost Rishi Sunak's party dozens of seats.
Mr Johnson said: "If you vote for Alice you will avoid handing Labour a majority, even perhaps a super-majority, to take this country backwards with higher taxes, more wokery, kowtowing to Brussels again and of course more illegal immigration. Don't let Starmer get away with it."
Mr Johnson's message seemed designed to appeal particularly to voters who may be considering backing Mr Farage's party.
It may be less tailored to London where the Tories are fighting to avoid losing around a dozen seats.
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Denne historien er fra June 18, 2024-utgaven av Evening Standard.
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