Theresa May’s local authority is prepared to spend £50,000 on a judicial review if her government approves the expansion of Heathrow next week, documents released on Monday reveal.

The papers underline the scale of resistance that the prime minister will face from residents in her Maidenhead constituency, which she has represented since 1997, if she agrees to allow the third runway to go ahead.

Papers published before a crunch council subcommittee meeting on Thursday warn that the plans would have a “significant potential impact on the quality of life” of people living in the area, where there is widespread support for Gatwick to get the runway instead.

Simon Dudley, the Conservative leader of Windsor and Maidenhead council, told the Guardian he was determined to protect residents from the “adverse consequences of an expansion of Heathrow airport”, regardless of May’s links to the area.

He said Thursday’s meeting was about setting in place the next steps in the legal process. “We have been elected on a manifesto promise, and we respect the views of our residents, who are firmly in favour of expansion at Gatwick,” he said.

It comes amid growing expectation that May’s government will accept the recommendation of Sir Howard Davies’s airports commission to expand Heathrow rather than Gatwick, with cabinet ministers telling the Guardian that the west London plans offered much higher-value returns than the alternative.

Before a decision expected next week, Dudley will be asking for colleagues to agree to a budget that is £20,000 more than originally suggested in preparation for a judicial review of a government approval.

His council has teamed up with three others, Hillingdon, Richmond and Wandsworth, who are particularly angry about the possibility of Heathrow expansion after the previous prime minister, David Cameron, made a “no ifs, no buts” promise that it would not happen.

The documents released on the council’s website on Monday evening reveal legal correspondence between the group and various government ministers.

Most recently, lawyers representing the local authorities wrote to the government’s airport capacity directorate on 30 September 2016 calling for a consultation “in advance of any decision”, claiming that consulting councils afterwards would represent a “sham”.

The letter claims that the process by which ministers have taken the decision has been “shrouded in mystery” and warned that the “lack of openness and transparency is of great concern”.

Specifically, the letter argues that government work on air quality was promised but has not been released, and says that EU limits on nitrogen dioxide concentrations remain binding.

“There is nothing in the public domain to suggest that expansion at Heathrow has been assessed against the government’s air quality plan, which is in any event subject to a judicial review challenge,” it warns. It also warns that plans would be unlawful without a better understanding of the impact on air and noise pollution.

The council is particularly concerned about residents who are currently not under flight paths, but would be under the new plans.

The lawyers say that poor air quality is known to have a worse effect on the young and elderly, as well as those in lower socioeconomic groups.

“A decision which favours Heathrow now, without consultation and a proper analysis of the health and equality impact of locating a third runway at Heathrow, would be in breach of the public sector equality duty,” they add.

Moreover, they say, the mitigating factors being promised by Heathrow, such as a ban on night flights, are “expressed in vague terms which cannot meaningfully be assessed or considered”.

A Heathrow spokesperson said: “Local council opposition has significantly decreased since Heathrow expansion was last on the table in 2009 from a group of 21 councils to four. Heathrow expansion is the only option being considered for expansion with any local political support – with four local MPs and two local councils backing our plans.

“It will be for the government to decide whether to go ahead with our proposals to add up to 25,000 flights per year from 2021 to underserved long-haul markets and domestic connections supporting Britain to make a success of Brexit.”