Zac Goldsmith, the MP for Richmond, has said he is ready to resign as a Conservative MP if Theresa May gives the go-ahead for a third runway at Heathrow.
And in a shock development his local Conservative group agreed to back him in a by-election over an official Tory candidate.
“They see it as a referendum via the ballot box on Heathrow”, an insider told the Evening Standard following a meeting of local party officials.
A fellow Conservative MP, Tania Mathias, who represents Twickenham, is also prepared to ignore the party’s rule book and actively campaign for a non-Conservative candidate.
Dr Mathias said: “Zac is doing the honourable thing by committing to his pledge and I fully support him in that.”
Mr Goldsmith, a prominent environmentalist who has long opposed expansion at Heathrow, has consistently said he would resign if his party backed a third runway.
He made opposition to Heathrow expansion a core plank of his campaign to be Mayor of London. Sadiq Khan, who defeated Mr Goldsmith in the contest, also favours expansion at Gatwick over Heathrow.
The shock move by Richmond Conservatives could cause a headache for the Prime Minister, who faces the prospect of seeing her parliamentary majority of 12 cut by one.
She will also be wary of losing the seat should the Conservatives field a candidate against Mr Goldsmith, thus potentially splitting the Tory-learning vote and handing the seat to the Liberal Democrats.
It comes as the Prime Minister told Cabinet colleagues they would be free to oppose the Government’s official position on airport expansion, suggesting it is Heathrow rather than Gatwick that will be given the go-ahead.
Both Boris Johnson, the Foreign Secretary, and Education Secretary Justine Greening have constituencies near Heathrow flight paths and are fierce opponents of expansion at the airport.
A small group of ministers, not including Mr Johnson or Ms Greening, are expected to make a final decision on airport expansion next week.
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