Conservative MP Zac Goldsmith has resigned, forcing a by-election in his constituency in a protest against the government's backing for a new runway at Heathrow Airport.
The Richmond Park MP and ex-London mayoral candidate has long campaigned against such an expansion.
Mr Goldsmith, who called the Heathrow decision "catastrophic", will run as an independent candidate.
Transport Secretary Chris Grayling has ruled out any Heathrow rethink.
The Treasury confirmed the formal process by which an MP steps down had been triggered, with the Richmond Park by-election thought likely to take place in December.
Mr Goldsmith is to take part at a meeting in his constituency, where he is expected to outline his plans.
Speaking in the House of Commons after Mr Grayling announced the Heathrow plans, Mr Goldsmith said: "The government has chosen a course that is not only wrong; it's doomed.
"It is wrong because of the million people who will directly suffer on the back of the environmental harm this project unavoidably produces, and doomed because the complexities, the cost, the legal complications mean that this project is almost certainly not going to be delivered."
He predicted Heathrow would be a "millstone around this government's neck for many many years to come".
But Mr Grayling told the House of Commons: "The prime minister has been very clear that she does not want to force - and indeed I don't think the public would expect us to force - MPs with long-standing principles of disagreement over this to go against their own views."
Mr Goldsmith held his south-west London seat in 2015 with a 23,015 majority. The Liberal Democrats, who held the seat until 2010, finished second.
