MI5 was scathing about American coastguards’ failure to attack a Nazi U-boat involved in a bungled World War II sabotage operation against the US, previously secret files show.

The German submarine ran aground on a sandbar as it dropped off four agents on the coast of Long Island, New York, on a top-secret mission to strike terror into America by destroying targets across the country.

A young coastguard interrupted the would-be saboteurs as they buried supplies on the beach – but the leader of the spies, George Dasch, persuaded him to walk away and leave them by giving him 300 dollars.

Lord Rothschild, head of MI5’s counter-espionage section, was sent to the US to prepare a report about the botched Nazi operation for the security agency’s director-general, David Petrie.

He wrote: “The submarine which landed one of the groups got into difficulties during the landing operation and went aground.

“It was only owing to the laziness or stupidity of the American coastguards that this submarine was not attacked by US forces.”

The June 1942 Nazi sabotage mission, named Operation Pastorius after one of the first German settlers in the US, was a disaster.

The plot was foiled when Dasch rang up the FBI to say he was a saboteur and wanted to speak to the bureau’s director, J Edgar Hoover.

This request was refused but he was interviewed at length by the FBI’s head of counter-sabotage and betrayed his fellow agents, who were quickly rounded up.

Lord Rothschild noted that Operation Pastorius was extremely well-planned, papers released by the National Archives today reveal.

“This sabotage expedition was better equipped with sabotage apparatus and better trained than any other expeditions of which the Security Service has heard,” he wrote in his January 1943 report.