The prospect of Heathrow Airport opening a third runway is a “fantasy” and should be “consigned to the dustbin”, according to foreign secretary Boris Johnson.

Bosses at the airport argue that its much-disputed expansion would result in 100,000 new jobs in the UK and fuel £56 billion of growth.

However, the former London mayor has opposed Heathrow’s third runway for years and has backed a new report which outlines potential risks to the taxpayer.

“Heathrow expansion is the wrong choice, and if it is chosen, it simply won’t get built,” Mr Johnson said.

He explained that the mammoth costs and risks attached to delivering the new runway render it “undeliverable”.

Mr Johnson added: “While we are finding this out, our international competitors will be further extending their competitive advantage over us. We need a better solution.”

Meanwhile, Heathrow CEO John Holland-Kaye believes that expansion would create a stronger British economy and secure jobs outside of London and the south-east.