Greg Hands quit as a trade minister in Theresa May’s government to enable him to oppose the expansion of London’s Heathrow Airport in a House of Commons vote next week.

Hands promised in last year’s U.K. general election that he would oppose the building of a third runway at the airport, to the west of London. He represents the district of Hammersmith and Fulham, which is under the flight path.

“As the Government will be whipping the vote on Monday, this means I am resigning from the Government,” Hands said Thursday on Twitter. “It has been an honour to serve the Prime Minister (and her predecessor) for the last 7 years and I wish the PM & the Government every continuing success.”

Hands quit because rules of collective responsibility dictate that all ministers should vote with the government.

Foreign Secretary Boris Johnson, another outspoken critic of Heathrow expansion, will avoid that dilemma because the vote has been scheduled on Monday, when he’ll be in Luxembourg for a meeting of the European Union’s Foreign Affairs Council.

Labour has given lawmakers a so-called free vote on the issue, in part because economy spokesman John McDonnell—whose district is next to the airport—is opposed to the plan, while the party’s trade union funders support it.