The resignation of junior trade minister Greg Hands from government in protest over Heathrow third runway now puts pressure on arch critic Boris Johnson ahead of a vote by MPs on the scheme on Monday.
The foreign secretary and MP for Uxbridge and South Ruislip is a long-standing opponent of the third runway and backed an alternative airport to be built in the Thames estuary when he was mayor of London.
Hands, who represents the Chelsea and Fulham constituency under the Heathrow flight path in London, said he had pledged to oppose the new runway at the 2017 election.
Johnson will reportedly be out of the country on Monday when MPs are asked to make the crucial vote on the third runway.
Meanwhile, the government confirmed that it is prepared to intervene to reserve slots at Heathrow for domestic flights if expansion goes ahead.
Public Service Obligations (PSOs) will be put in place to protect routes if required, which would also exempt them from Air Passenger Duty.
It would mark the first time that flights from Heathrow to regional airports have been safeguarded.
Reserving flights specifically from regional airports to Heathrow would ensure direct links to the UK’s hub airport and open up vital connections with the rest of the world, the DfT said yesterday.
The government wants about 15% of new capacity at an expanded Heathrow to be used for domestic flights.
Scotland can expect to benefit from an extra 100 flights a week to and from Heathrow if a third runway is built.
Hands’ resignation was described by the backers of alternative extended northern runway scheme Heathrow Hub as “unnecessary and a direct consequence of the Department for Transport’s flawed decision-making”.
A spokesman said: “Our extended runway was specifically designed to take account of local concerns and to be politically palatable.
“We met Mr Hands in 2015 and he made clear he would not have to oppose it. Heathrow’s third runway is disastrous for Hammersmith and Fulham residents, bringing thousands of them into the noise envelope for the first time and reducing respite.
“By contrast, our simple extended runway plan can be built in phases and we have proposed alternating flight paths offering more respite than now.”
The spokesman added: “The resignation of Mr Hands, the pressure now on the foreign secretary and the upcoming legal challenges by councils in London and the mayor of London Sadiq Khan are a direct result of the bungled process run by the Department for Transport.
“Not only did it allow Heathrow to veto our cheaper, quieter, quicker scheme, apparently because its shareholders would make less money, it has made serious mistakes over our capacity.
“We have repeatedly asked for our scheme to be included in the [aviation] National Policy Statement but this common-sense suggestion has not been taken on board.”
Heathrow Hub has complained to the Competition and Markets Authority and preparing to judicially review the National Policy Statement if it is designated following the vote in parliament.
Aviation minister Baroness Sugg said: “Heathrow is situated in the south-east of England, but the benefits of expansion will be felt throughout the UK.
“About 15% of the new capacity will be used for flights to destinations within the UK or Crown dependencies.
“And now we are going even further by confirming that the UK government will act to protect domestic flights from Heathrow for the first time.”
Routes could also be reserved for specific times to ensure they deliver services for passengers when they most need them, for example early morning flights which will cater for businesses.
Additional capacity at Heathrow is also expected to encourage competition between airlines, bringing down prices for British passengers, the DfT claimed.
It cited easyJet and Flybe as already outlining new routes they would expect to serve from an expanded Heathrow. EasyJet also confirmed earlier this month that it intends to start new flights to UK airports and introduce competition on existing routes, lowering fares by up to 30%.
However, the Scottish National Party is demanding assurances that Scotland would benefit before supporting the third runway proposal at Monday’s vote.