Boris Johnson has travelled to Afghanistan on the day MPs are set to vote on Heathrow Airport expansion - a plan he opposes.
The foreign secretary, who pledged to lie down in front of bulldozers to block a third runway being built at the west London airport, defended missing the vote rather than quitting the frontbench by saying resigning would "achieve nothing".
Government frontbenchers are bound by "collective responsibility" to support official policy.
Most Tory MPs are expected to back Prime Minister Theresa May and vote for Heathrow expansion, but the former London mayor will be abroad.
The Foreign Office initially would not give any further details for security reasons, but later confirmed Mr Johnson had flown to Afghanistan's capital Kabul for talks with the country's president Ashraf Ghani, chief executive Abdullah Abdullah and deputy foreign minister Hekmat Karzai.
The foreign secretary also met the head of the NATO mission in Afghanistan, UK troops stationed in the country and a group of schoolgirls who are getting access to sports education through UK funding.
Mr Johnson's visit came after a recent ceasefire between government and Taliban forces during the Muslim festival Eid.
He said: "At this important moment when Afghan-led efforts towards peace and a political settlement have gained considerable impetus, I was proud and inspired to be in Kabul to see how the UK is working in support of the Afghan government to achieve this goal.
"In my meetings with President Ghani and chief executive Abdullah, I welcomed their historic offer of unconditional talks earlier this year, and their determined recent efforts towards a political process with the Taliban.
"I urge all countries with influence in Afghanistan, especially in the neighbourhood, to use it constructively at this crucial moment."
Mr Johnson added "there is still a lot" the UK could contribute to NATO's mission in aiding Afghan security forces, amid a US and NATO request for more help, as he revealed the government "will be taking a decision very soon".
After an initial period of silence, a letter from Mr Johnson to his Uxbridge and South Ruislip constituents defending his absence from Monday's vote was obtained by the Evening Standard.
He wrote: "My resignation would have achieved absolutely nothing."
Mr Johnson also predicted Heathrow's third runway would never be built.
It follows fellow London member of parliament Greg Hands quitting as trade minister to vote against the expansion .
"Tonight is about fulfilling that pledge in exactly that way," the Chelsea and Fulham MP told Sky News on Monday.
"I think the impact of Heathrow expansion will be considerable and detrimental in my constituency - more widely I think it's the wrong decision."
He was abroad when he quit as a minister, but landed back in London today to make the vote.
Tory MP Justine Greening, who left the government in January and also opposes Heathrow expansion, tweeted: "Great you're back Greg! I wouldn't want any long-term MP campaigners against Heathrow expansion to miss their chance to represent their community. #commitment #bulldozer."
Earlier, transport secretary Chris Grayling called the vote "the biggest transport decision in a generation".
He also admitted he "didn't know where" Mr Johnson was.
But Mr Grayling added: "We all fought a general election on the manifesto of expanding Heathrow Airport.
"But equally, where there are people who've got particular constituency issues, we've left them the freedom to carry on expressing the views we've always had."
Senior Tory backbencher Dr Sarah Wollaston has called on the foreign secretary to resign.
She said the decision to let Mr Johnson avoid a three-line whip in support of the Heathrow plan by "won't wash".
"I think this would be an opportunity for a colleague like Boris Johnson to actually put his money where his mouth is," Dr Wollaston said.
The vote on Monday evening is also set to expose divisions among the Labour ranks.
More than 40 Labour MPs said they would go against party policy and support the government.
It is officially opposed to the expansion but leader Jeremy Corbyn has allowed MPs a free vote on a measure that is supported by trade unions.
Some opponents have criticised the scheme on environmental, noise and financial grounds.
Friends of the Earth said it was "morally reprehensible" and would see the enlarged Heathrow emitting as much carbon as the whole of Portugal.