UK Transport Secretary Chris Grayling is to outline the details of Heathrow Airport’s controversial expansion plans today after years of delays.

Prime Minister Theresa May, who is chair of the economic sub-committee, is expected to support the airport’s proposal for a third runway before sending it to the full cabinet.

If approved, the plans will then be outlined to the House of Commons and MPs will have to vote on them before the end of the month.

Talks of expanding Heathrow Airport started nearly 20 years ago. The west London hub was given the greenlight in 2016, triggering concerns about the impact a third runway could have on the environment and nearby residents.

The issue has sparked a division among ministers, with former Transport Secretary Justine Greening claiming the plans were “ill-conceived” and Foreign Secretary Boris Johnson often reiterating his opposition to them.

It is thought May could hand MPs a free vote on the matter, though several ministers are concerned the move could become a “dangerous precedent” and force dozens of anti-Heathrow Conservative party members to halt the expansion.

There is further concern that Labour ministers will oppose the plans after the party’s manifesto claimed specific environmental conditions would have to be met in order for the proposal to receive support.

A Heathrow spokesman said: “The government is best-placed to comment on their timetables. What is clear is that the case for expanding Heathrow has never been stronger.

“We’re looking forward to the promised parliamentary vote on the final national policy statement before the summer recess enabling us to get on with delivering this critical project.”

Environmental campaigners have also complained about the expansion, suggesting it would breach the UK’s legal limits on air pollution, as well as increase noise pollution with 700 additional planes a day.

Earlier today, a coalition of local authorities said it would start a legal fight over Heathrow’s plans if the government fails to alter them.

Windsor and Maidenhead borough council leader Simon Dudley said four authorities have partnered with Greenpeace in a bid to oppose the government in case of approval.

Dudley said: “We will look at the proposals to see whether our significant concerns have been addressed.

“If they have not, whether or not Windsor and Maidenhead Council is involved, there will be a legal challenge”.

Heathrow’s owners said that the airport is virtually full and a new runway, which could become operational between 2025 and 2030, would boost its capacity from 85.5 million to 130 million passengers, create 60,000 new jobs and generate about £70bn in total economic benefits by the 2050s.