Security was being ramped up Thursday outside Windsor Castle (search), where Prince Charles will marry his longtime love Camilla Parker Bowles on Saturday.

The castle will be protected by police on horseback, plain-clothes officers and sharpshooters on rooftops. In order to thwart the paparazzi, a large green screen was being placed at the top of the main castle wall. The blessing and a reception for 750 guests will take place in the castle after the civil ceremony.

A reporter for British tabloid the Sun said he was able to get inside the castle without a pass or an appointment in a van that contained a brown package with the word "bomb" written on it.

The Sun said the reporter came close to apartments in the castle reserved for Queen Elizabeth II (search) and the chapel where the blessing ceremony will take place, after Charles and Parker Bowles marry in Windsor's town hall.

British police said they were investigating the report.

Royal security is notoriously leaky. A comedian dressed as Usama bin Laden crashed Prince William's 21st birthday party at Windsor Castle in 2003. Later that year, a reporter from the Daily Mirror got a job as a servant at Buckingham Palace and took pictures of the royals' living quarters.

In September, a protester dressed as Batman climbed onto a ledge on the front of Buckingham Palace and remained there for several hours.

On Tuesday, the Sun reported that two intruders broke into the private area of Windsor Castle on Sunday.

FOX News' Catherine Herridge and the Associated Press contributed to this report.

Fox News Channel's parent company, News Corp., is the parent company of the Sun.