Footage from the News of the World reporters' tour of the Palace - video courtesy News of the World
Buckingham Palace has suspended a chauffeur after undercover reporters claimed to have gained access to highly sensitive areas of the building.
A palace spokeswoman told the BBC that an investigation would be carried out into allegations that Brian Sirjusingh was paid £1,000 to give them a tour.
Two reporters from the News of the World newspaper are said to have been waved inside, without security checks.
It is alleged one of them sat in the Queen's state Bentley car.
BBC royal correspondent Peter Hunt said he understood Mr Sirjusingh was a pool chauffeur - one called when the dedicated royal chauffeurs are unavailable.
The News of the World reports the journalists posed as Middle Eastern businessmen and were waved into what were supposed to be secure areas of the Queen's home.
According to the paper, the men were led past a police checkpoint and a sign demanding to see identification, and into the royal garage.
It could easily have been a terrorist walking into the palace and planting a bomb
Robert Jobson News of the World
Once inside, Mr Sirjusingh showed them several vehicles used by members of the royal family and allowed one reporter to sit in a Bentley used to transport the Queen on state occasions, the paper said.
A Scotland Yard spokesman said: "We are naturally concerned about the issues raised by this story and are liaising with palace officials about their staff security arrangements."
The newspaper's royal editor, Robert Jobson, told the BBC that lessons should have been learnt from previous security breaches.
He said: "There have been a number of security breaches at the palace over the years but this is right up there in terms of being a flagrant breach of the security.
"They should have been checked as they walked in but they weren't and therefore it could easily have been a terrorist walking into the palace and planting a bomb in the car rather than the News of the World exposing the poor security of the palace."
Buckingham Palace said it was taking the matter "very seriously".
Dai Davies, who was head of royal protection in the 1990s, said he thought the incident was "pretty serious".
No-one should be allowed in without absolute proof of identity
Dai Davies, former head of royal protection
"It seems almost impossible to believe that somebody could access this particular part of the royal premises with such ease," he told the BBC.
"It does seem to be a fundamental failure in system and supervision.
"No-one should be allowed in without absolute proof of identity and those identities should be pre-arranged and pre-checked."
In recent years there have been a number of high-profile royal security breaches.
In 2003, Daily Mirror reporter Ryan Parry spent two months working undercover as a Buckingham Palace footman.
He used a false reference to get the job despite unprecedented security surrounding the visit of US President George Bush to the UK.
The same year, during Prince William's 21st birthday, comedian Aaron Barschak set off six alarms and appeared on CCTV several times without sparking a response when he gatecrashed a party at Windsor Castle.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.