Labour says Boris Johnson should pay for an overseas trip that caused him to miss a crunch Heathrow vote, after it emerged it cost more than £20,000.
The bill for staff accompanying Mr Johnson - foreign secretary at the time - to Afghanistan in June was £19,366.
The Foreign Office would not reveal the costs for Mr Johnson, saying a figure would be published "in due course".
He was criticised at the time for missing the vote on a third runway, which he had long opposed.
Conservative MPs were told to back Heathrow expansion, and Mr Johnson would have had to resign from government if he had wanted to vote against.
He has since resigned over Brexit and is now a backbench MP.
Responding to a Freedom of Information Act request by Scottish investigative website The Ferret, the Foreign Office said the trip to Afghanistan had been a "timely opportunity" for the then foreign secretary to "take stock" of progress in the country.
Mr Johnson held talks with senior figures including Afghanistan's president, and met UK troops helping to mentor Afghan forces.
Last month, Defence Secretary Gavin Williamson announced the UK would send 440 more troops to Afghanistan to serve in non-combat roles.
Shadow chancellor John McDonnell, who also opposes Heathrow expansion, said Mr Johnson should use some of the money he earns as a newspaper columnist to cover the bill.
He added: "Boris Johnson, who once said he would lie down in front of bulldozers to stop a third runway at Heathrow, scuttled out of the country at the taxpayer's expense rather than honouring his promise to his constituents."