Metropolitan Police chief Sir Ian Blair has ordered an inquiry into claims journalists drove a van carrying a fake bomb into Windsor Castle's grounds.
The couple will have their marriage blessed at the castle chapel
The Sun said the van passed St George's Chapel, where the marriage of Camilla Parker Bowles and Prince Charles will be blessed this Saturday.
The "apparent security breach" raised serious concern, Scotland Yard said.
The report comes after it emerged that on Sunday two tourists scaled a fence and entered a private castle area.
Scotland Yard said Sir Ian wanted to establish the facts surrounding the latest report in Thursday's Sun.
The newspaper claimed it breached the castle's £5m security barrier with "breathtaking ease" and got to within a "stone's throw" of the Queen's apartments.
One of its reporters and a photographer say they drove up in a hire van with no security passes and no pre-arranged delivery time.
On board was a brown box marked "bomb" and the reporter says he had a fake delivery note.
After an attempt to check up on them failed, they were allowed to drive into the grounds - past the chapel where the royal couple will be blessed - and they were not searched, the paper said.
Scotland Yard said in a statement: "It's only right the facts are established before any action is taken against any person who may be culpable."
A spokeswoman for Buckingham Palace said: "Security is a matter for the police who have been asked to investigate."
Windsor Castle staff are already investigating how two men were able to enter one of its private areas last weekend.
Scotland Yard said the tourists were detected immediately and taken back to the public area but not arrested.
WINDSOR SECURITY BREACHES
Feb 2005: Conman Michael Hammond, who posed as a senior police officer to get into Windsor Castle, is jailed
June 2003: Intruder Aaron Barschak jumps on stage dressed as Osama bin Laden at Prince William's 21st birthday
Nov 2003: Reporter Ryan Parry, who got a job at Buckingham Palace, claims security at Windsor is worse
1994: Security review launched after a ceremonial sword is stolen from the castle museum
1994: A pair of Eton schoolboys scale the castle walls and trigger alarms
"The secure area of the castle was not breached at any stage", a spokeswoman said.
Meanwhile, on Wednesday, police said they hoped Buckingham Palace would contribute to costs caused by the change in the royal wedding date.
The switch, from Friday to Saturday because Prince Charles will be attending the Pope's funeral, may result in a big rise in security costs as extra police are drafted in on their days off.
Meanwhile, Prince Charles is preparing for his last official UK engagement before the wedding.
He is a patron of Breast Cancer Haven and will visit the charity's centre in Fulham, London, on Thursday.
The new wedding date and time - 1230 BST on Saturday - has also affected the Grand National at Aintree, with a change in start time from 1545 BST to 1610 BST.
Weathermen say it is set to rain on Saturday, with Windsor having quite chilly temperatures of 6C (43F), which is lower than the seasonal average of 12C (54F).