The Daily Mirror said Ryan Parry worked for two months at the Palace despite unprecedented security for the visit of US President George Bush.

Mr Parry, who had access to private suites, claims he could have assassinated the Queen or Mr Bush.

Buckingham Palace said it was conducting a full investigation.

Scotland Yard has put in place a £5m operation which will see more than 5,000 police on the capital's streets for the state visit, while hundreds of armed US security staff will also guard the president.

The Daily Mirror story includes pictures by photographer Phil Harris from inside the palace of the bedroom used by Mr Bush and his wife, The Belgian Suite.

There are also pictures of the Queen's breakfast table and the Duke of York's room.

Mr Parry wrote: "Had I been a terrorist intent on assassinating the Queen or American president George Bush, I could have done so with absolute ease.

"Indeed, this morning I would have been serving breakfast to key members of his government, including National Security Adviser Condoleezza Rice and US Secretary of State Colin Powell.

"Such is the shocking incompetence at the heart of the biggest security operation ever in Britain."

In August, Mr Parry responded to a job advertisement on a recruitment page of the Buckingham Palace official website.

On his CV, he left out any mention of his journalistic career and included one fake reference and a real one, the newspaper claimed.

Mirror editor Piers Morgan said the paper used "very basic subterfuge and got incredible access".

"To our surprise and then mounting horror we discovered that our man with no training, no experience at all, no real vetting was in very close proximity to the most important people in our country," Mr Morgan told BBC Radio 4's Today programme.

He described the reference checking as "scandalous" and "shoddy".

Mr Morgan alleged that someone from the palace rang Mr Parry's local pub where he had given a name as a character reference.

Mr Morgan said they had decided to pull Mr Parry out of the palace once the president arrived as "we did not want to genuinely compromise any ongoing security issues that might arise while [the president] is here."