When the SS Richard Montgomery set sail for Normandy in 1944, the threat posed by the Luftwaffe and U-boats loomed large.

The American Liberty-class cargo ship was carrying large quantities of munitions to France to consolidate the Allies’ D-Day gains, but a storm meant she never got further than the Thames.

Now, a shipwreck at the bottom of the river off Sheerness, Kent, the vessel has become a “sitting duck” for drone attacks by Russia and Iran.

Last year, aircraft were banned from flying over the wreck, which has 1,400 tons of explosives on board, because of fears that hostile nations were plotting to attack it, it can be revealed.

In October, it was reported that an ally of Vladimir Putin had told Russian state TV that Moscow should target the ship. Russia is believed to be behind a series of drone sightings near military installations across Europe.

The detonation of the Richard Montgomery could trigger “mass damage and loss of life” by sending a 5m-high tsunami towards Kent, Essex and Britain’s largest liquefied natural gas terminal on the Isle of Grain, previous government assessments have warned.

It is feared drones could topple the fragile masts of the “doomsday wreck” – which protrude ominously above the surface of the Thames – onto its incendiary cargo below.

The ship has a cocktail of explosives on board: high-explosive general-purpose bombs, fragmentation bombs designed to scatter shrapnel over wide areas, and incendiary devices, including white phosphorus bombs.

The cargo also contains thousands of bombs in the 1,000lb and 500lb ranges and bomb clusters, which hold multiple bomblets.

Any explosion could cause rapid flooding in North Kent and South Essex, overwhelming their flood defences.

Government sources admitted they were alert to the prospect of a drone attack on the sunken ship, which lies a few hundred metres from shipping lanes leading to the Port of Medway.

The no-fly zone was implemented after a government survey of the wreck was published last year. It identified “significant” deterioration in its condition.

Katja Bego, a senior research fellow in the Chatham House think tank’s Europe programme, said “a bad actor could smuggle more sophisticated drones, or rig with explosives readily available commercial drones, and … launch an attack from UK soil directly”.

She added: “The scary thing is that you do not need to be a highly sophisticated state actor to launch a sabotage attack using commercially-available drones. The biggest hurdle is acquiring the explosives – and finding a good drone pilot.”

A nautical exclusion zone banning vessels from the vicinity of the wreck was extended this week to allow for a new survey. The no-fly zone, introduced in June, is the first permanent airspace restriction over the Richard Montgomery since she ran aground in August 1944.

Half the munitions were removed during salvage efforts after the grounding, but the rest of the cargo was left on board when the ship started to flood through a crack in the hull. The explosives remain untouched because of fears that a new salvage operation would set them off.

The risk posed by drones was one reason why all aircraft were banned from flying within a one-nautical-mile radius of the wreck for “reasons of public safety” last summer.

Major Andrew Fox, a retired paratrooper who served three tours of Afghanistan, said: “The wreck is at risk of drone sabotage because it is a fixed, conspicuous target.”

Prof David Alexander, an expert in emergency planning and risk management, described the Richard Montgomery as “a sitting duck”.

The Government has said it wants to remove the vessel’s three masts “as soon as possible” to prevent their collapse, but the project has been repeatedly delayed since it was first announced six years ago.

Marco Wyss, professor of international history and security at Lancaster University, said saboteurs would probably be “proxies” hired for the operation.

He said: “It’s almost equivalent to an act of war, depending on how it’s interpreted. If Russia or Iran … did it, then they would also use perhaps a more professional kind of proxies because it’s a more complicated operation.”

On Tuesday, Lord Beamish, the chairman of Parliament’s intelligence and security committee, warned that Russia and Iran were paying people in Britain to launch “proxy” attacks.

He told BBC Radio 4’s Today programme: “In our report in 2025, we highlighted the Iranian regime, whether it be the IRGC (Islamic Revolutionary Guard Corps) or Iranian security services, do attack dissidents, people who criticise the regime, and target the Jewish community.

“And they do that increasingly not directly, but through proxies. What you’re dealing with here is not necessarily just organised crime groups but also people who are just paid.”

The Civil Aviation Authority (CAA) said the no-fly zone was introduced to protect passenger airlines flying into London City and Biggin Hill airports.

A spokesman for the CAA said: “Restricted airspace was introduced last year above the World War II wrecked vessel, SS Richard Montgomery.

“This restriction, requested by the government, is to ensure the safety of all inbound air traffic and to prevent any potential incidents from the unexploded bombs on the ship. This impacts aircraft routes into London City and Biggin Hill airports.”

However, Stephen Miller, a former Ministry of Defence explosives engineer, said planes flying overhead were “not at risk” from the wreck. He said: “I believe the no-fly zone has been put in place to protect the wreck, and the … mass use of drones by the public is the most likely reason.”

The Department for Transport said the wreck remained in “stable condition” but that drones “must not fly” above it. A spokesman said: “Our priority will always be to ensure the safety of the public and to reduce any risk posed by the SS Richard Montgomery.

“We have been clear that pilots and operators, including drones, must not fly in a limited area around and above the site, which is standard for no-fly zones across the country. The condition of the wreck remains stable, and the site is rigorously monitored 24/7.”