Unexploded wartime bombs deteriorating within a sunken ship could put Boris Johnson’s controversial plans for a Thames Estuary airport up in smoke.

The wreck of the SS Richard Montgomery, which contains 1,400 tons of deadly munitions, has been grounded on a sandbank off the Isle of Sheppey since August 20, 1944, and is rarely examined for fear it might self-destruct and kill thousands of people.

Already described as a “major obstruction” in a study commissioned by Mr Johnson, a new report into the safety of the vessel by the Marine and Coastguard Agency has cast further doubt on the Mayor of London’s much-criticised plans for a new airport to replace Heathrow.

It says: “While much of the hull appears to be deteriorating relatively slowly, one of the main concerns is that these specific areas of accelerated deterioration might lead to structural collapse or allow the escape of munitions.

“Although some munitions studies have been carried out, these are not sufficient to predict with any certainty what the effect of significant structural collapse would be on the munitions cargo.

“There may come a point when the risks associated with non-intervention will become greater than the risks associated with a carefully-planned intervention operation.

“While significant structural collapse does not appear to be imminent, surveys suggest this prospect is getting closer.”

Mr Johnson believes a £40 billion floating airport could be built in the Thames Estuary within 10 years, but his plans have been blasted by politicians, environmentalists and members of the public alike.

Sittingbourne and Sheppey MP Gordon Henderson (Con) is among those who oppose the ‘Boris Island’ plan, and said the location of the SS Richard Montgomery was one of many reasons why it should be rejected out of hand.

He said: “The ship is not something that’s always on my constituents’ minds, but it’s on my mind because I feel I have a responsibility to ensure they are not put at risk in any shape or form.

“The location of the Montgomery is a small part of the overall objections we have to the airport plan. The biggest is that if Boris Johnson believes it’s necessary to close down Heathrow and move it to the Thames Estuary, then it must be because he believes Heathrow has a negative impact on the quality of life of people in west London.

“If that’s the case then why on earth would we want the same problems? If we forget that primary objection then we lose that very powerful argument.”