The Conservatives will not fight the by-election triggered by former Tory MP Zac Goldsmith, who has quit in protest over Heathrow expansion, a party spokesman said.
Mr Goldsmith said the decision to push ahead with a third runway was "catastrophic" and announced he will stand as an independent.
But the Richmond Park and North Kingston constituency was held by the Liberal Democrats until 2010 and splitting the Tory vote could allow them to retake the seat.
The Conservatives said they "understand" Mr Goldsmith's position and praised him as a "hard-working champion" for the area.
A spokesman said: " The Government is taking decisive action in the national interest to secure the UK's place in the global aviation market - securing jobs and business opportunities for the next decade and beyond.
"That's why we have delivered on our manifesto commitment to respond to the Airports Commission's final report and have announced our support for the first full-length runway in the South East since the Second World War.
"We disagree with Zac about the need for a by-election in light of this decision, but understand his position. He has been a hard-working champion for the people of Richmond Park - as we know he will continue to be if they re-elect him as their Member of Parliament - so we will not be putting up a candidate against him at this by-election."
Mr Goldsmith, who had long threatened to resign if the Government backed the expansion plan, said c onstituents felt "let down" after the promises made by the Tories under David Cameron had not been kept, he said.
The high profile environmentalist insisted the fight to stop expansion was not over and said the by-election would be a "chance to send a message to government".
He said: "I promised you if my party won the election, the third runway would be scrapped. And I wasn't making it up.
"My party leader - the then leader of the Opposition - made that same promise directly to us. He came here and told us: 'No ifs, no buts, there will be no third runway'.
"It was music to our ears. But not everyone believed him, because people assume the worst in politicians. So I followed his promise with my own. I told you that if my party changed its position, I would trigger a by-election and give you a chance to vote again.
"There was no small print. No expiry date. No ambiguity. It was a simple promise. And it mattered. I know it mattered, because the thought of Heathrow expansion fills most of my constituents with dread.
"That's why my party's promise mattered. It's why my promise mattered. And it's why so many people in our community feel so let down today."
Mr Goldsmith said the Government had chosen the "most polluting, most disruptive, most expensive option" but it had "also chosen the option with the least chance of being delivered".
"The sheer complexity, cost and legal difficulties mean it is unlikely ever to happen," he said. "It will be a millstone round this Government's neck for years."
The Lib Dems are likely to pour resources into the by-election battle after heavily cutting the Tory majority in the Witney by-election last week in a contest triggered when Mr Cameron stood down as an MP.
Leader Tim Farron said: "We are looking forward to building on our result in Witney and taking the fight to Richmond Park.
"A victory for the Liberal Democrats would put huge pressure on Theresa May to abandon her plans for Heathrow expansion and an economically disastrous hard Brexit."