The Lib Dems are claiming that it would be impossible to build the "Boris Island" airport in the Thames estuary because of the presence of a sunken American warship from the Second World War, which is packed with high-grade explosive.
The SS Richard Montgomery sank in the estuary more than 60 years ago, and has long been known to contain more than 1,400 tonnes of TNT.
The latest annual study from the Maritime and Coastguard Agency, published this week, suggests the hull of the wreck is still deteriorating, and the risk of an explosion, while remote, cannot be ruled out.
This, claims Lib Dem MP Julian Huppert, "must surely put an end to the bonkers idea of building an airport in the Thames Estuary."
“Just last month the Royal Navy had to destroy a wartime mine found in the Thames Estuary because of the ‘significant risk to public safety," claims Huppert.
The maritime and costguard agency does annual surveys of the wreck - the latest were published on Tuesday. Department for Transport sources said it was a significant amount of explosive, but added that it was common knowledge that the wreck contained the TNT.
The report indicated that while the rate of deterioration was not accelerating, some parts of the ship were impossible to accurately assess without bringing in more advanced sonar technology.
Boris Johnson has threatened to resign as Mayor of London if the government U-turns and decides to build a third runway at heathrow airport. He has proposed a new airport, either on the Thames Estuary or even on a reclaimed island between Kent and Essex.
“If this cargo ship was disrupted by construction the explosion would be 2000 times larger, it would blow out every window in Sheerness, and create a 16ft wave just outside the capital," claimed Julian Huppert.
“The last time we tried to move a similar wreck it exploded. Without a credible plan to deal with this mess there’s no way Boris’ plans will ever get off the ground.”