Diana, Princess of Wales, was the subject of an FBI investigation after an agent was told someone had mailed a bomb as a wedding present according to de-classified documents.

The investigation was opened when a woman phoned from Annapolis, Maryland, and said she had overheard a conversation that a man had mailed a bomb to the Prince and Princess of Wales ahead of their wedding in July 1981, the Daily Telegraph reported.

The investigation was subsequently dropped when the man who made the alleged threat said he had spoken sarcastically according to the FBI Vault details.

Princess Diana was killed alongside his boyfriend Dodi Al-Fayed in a car crash in Paris in 1997.

The files are among thousands released on a new FBI website called 'the Vault', which contains files dating back decades that have been scanned from paper into digital copies and classified into subjects from Unexplained phenomenon to Foreign counter-intelligence.