LONDON (REUTERS) - British Prime Minister Boris Johnson said on Monday (Dec 9) he would try to find a way to honour his promise to oppose Heathrow airport expansion by lying down in front of bulldozers but that it might be difficult for him.

The British parliament has approved an expansion plan for Heathrow, including building the first full-length new runway in the London area for 70 years, but the plans are being challenged in the courts.

Before he won the top job, Johnson opposed Heathrow expansion and promised to lie down before bulldozers to stop it.

"On Heathrow, it's a private sector project which is still to satisfy its strict legal obligations on air quality and noise pollution," Johnson told LBC radio.

"I don't see much sign of any bulldozers yet... I would have to find some way of honouring that promise. It might be technically difficult to achieve... Let's wait and see when the bulldozers arrive." Asked about a HS2, a proposed high-speed rail project to improve links from London to central and northern England, Johnson said the cost would be more than 100 billion pounds (S$174 billion).