Supported by a coalition of local authorities, he believes the expansion would breach European Union laws on pollution and has vowed to sue if there is a basis for legal action.
Litigation could delay plans to have the runway completed and operational by 2030.
Johnson has pledged ВЈ15,000 towards the cost of a legal challenge if lawyers give the go-ahead, reported the Sunday Times.
His spokesman said the business case for expansion did not outweigh вЂњconcerns for the local environment in terms of noise and air qualityвЂќ.
Approval for the third runway is expected next month.
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