The head of the UK Environment Agency has cast doubt over the viability of a third runway at Heathrow and said the government’s go-ahead for the project “defies logic”.

Environment Agency chairman Lord Smith says there is “a very big chance” the runway will never be built and said ministers had “made a mistake”.

The comments may prove significant as the Environment Agency could block the development if ground-level pollution or noise limits at Heathrow are breached.

Lord Smith told the Financial Times: “The opposition will make it difficult for [airport operator] BAA to proceed with extensive planning and design work.” He also questioned whether aircraft engines will become cleaner as rapidly as the government projects. “I have my doubts,” he said.

As a member of the government, Smith was responsible for tourism as Secretary of State at the Department of Culture, Media and Sport.