Tuesday 18 October 2016 1:40 pm

Boris Johnson will be allowed to protest if the government opts to expand Heathrow next week

Foreign secretary Boris Johnson would be allowed to speak out against any plans to expand Heathrow under highly unusual plans revealed by Downing Street today.

Theresa May's office said earlier today that a verdict on whether to expand either Gatwick or Heathrow airports would be reached on Tuesday next week.

However, while there will be a government position, May has offered an olive branch to ministers by allowing them to publicly push back against that line.

Although Downing Street initially suggested ministers would be expected to fall in line at a later date, it has now clarified that limitations will be on what they can saw, and where.

For example, ministers cannot speak in the House of Commons against the government's preferred option for expansion, and they will also be blocked from launching a "campaign" against it, or questioning the decision making process.

Final parliamentary approval for any airport scheme is not expected to come through until Winter 2017/18.

The offering from May reflects the strength of opposition within the government for Heathrow expansion, with a growing expectation that the government will opt for that site over Gatwick.

Johnson led a campaign against Heathrow expansion as London mayor, and said in his speech upon being elected as Uxbridge MP that he would stand "in front of those bulldozers and stop the building, stop the construction of that third runway".

However, he could also be joined by education secretary Justine Greening, whose West London constituency of Putney, Roehampton and Southfields lies under flightpaths for the airport.

Greening has regularly campaigned against Heathrow expansion, and told the Telegraph over the summer that trying to expand the airport “is like trying to build an eight bedroom mansion on the site of a terraced house”.

The constituencies of May and chancellor Philip Hammond would also be substantially affected by any decision, although both have been keen to avoid pronouncements on the topic.

And even outside of the government, support for Heathrow could create further upset – Richmond MP Zac Goldsmith, who lost out in the summer's London mayor election to Sadiq Khan, has previously vowed to quit if a new runway at the airport was given the greenlight.