Boris Johnson must quit the cabinet over the government's decision to give a third runway at Heathrow Airport the go-ahead, a co-leader of the Green Party has demanded.
Jonathan Bartley told IBTimes UK the foreign secretary and former Mayor of London should make the move despite Number 10's promise to allow ministers to express "personal views" for a "limited time" over the decision.
"A free vote isn't enough, he will still be muzzled and he would still not be able to campaign against Heathrow expansion," Bartley said.
"He's nailed his colours to the mast – he said he would campaign against it and the only way to do that is to resign."
Richmond Park MP Zac Goldsmith is expected to go one step further and quit the House of Commons over the decision.
The environmentalist will reportedly tender his resignation to the Richmond Park Tories today.
Goldsmith, who was first elected to parliament in 2010, told IBTimes UK last week: "Heathrow has always been the default position of successive governments because the company used to be a monopoly with total access to government, and many of those relationships continue today.
"But in addition to the noise, cost and pollution, a green light for Heathrow would mean more years of dithering and delay, and for that reason there is still a strong chance it will be rejected."
The dramatic move would trigger a by-election for the London seat. The former Mayor of London hopeful currently holds a majority of more than 23,000 over the Liberal Democrats.
But Goldsmith could split the Tory vote if he ran as an independent candidate.
"[The by-election] is clearly going to be a national issue around airport expansion, the whole country is going to be focused on it," Bartley said.
"There needs to be a party there which says very, very clearly: no Heathrow expansion, but not Gatwick expansion either.We need to be taking our climate change commitments very, very seriously."
The Richmond Park vote would follow the Witney and West Oxfordshire by-election, which saw the Conservative majority slashed from more than 25,000 to just over 5,700.
The government, meanwhile, is expected to launch a public consultation on a third runway at Heathrow ahead of a parliamentary vote in the winter of 2017/18.
The SNP backs a third runway at Heathrow, while Jeremy Corbyn is expected to give Labour MPs a free vote on the issue.
Labour's Mayor of London Sadiq Khan is opposed to the move as well as the Liberal Democrats.