A man posing as a police officer was arrested in the grounds of Windsor Castle last night.
A woman, thought to be his girlfriend, was also arrested.
Police said no member of the British royal family was present at the time.
"Both are currently in custody at a London police station," a police spokeswoman said. "At this stage it is not believed that security was breached."
Sources said the man, who posed as a plain clothes police officer, was trying to gain free entry to the public part of Windsor Castle.
Security at the castle 20 miles west of London was raised in May last year when self-styled "comedy terrorist" Aaron Barschak managed to gatecrash the 21st birthday party of Prince William that was being held there. He was arrested without incident.
Royal security suffered another public set back six months later when a reporter for the Daily Mirrorgot a job as a servant in the royal household ahead of US President George W. Bush's state visit.
The royal family appointed a new chief of security ten days ago.