A man arrested for allegedly impersonating a police officer at Windsor Castle is also being questioned about suspected hoax calls.
The castle is a major tourist attraction
The 33-year-old man is in hospital because of a pre-existing medical complaint, Scotland Yard said.
He is in custody along with his 29-year-old girlfriend who was arrested on suspicion of deception.
No members of the Royal Family were at the castle at the time and police say the pair did not breach security.
A spokesman for Scotland Yard said the man would be interviewed in connection with telephone calls made in May to four police forces across south-east England where the caller claimed to be a police officer.
The calls, which were considered to have been serious attempts to misdirect police resources, were already being investigated, the spokesman said.
Home Secretary David Blunkett is being briefed on Monday's incident.
Earlier Scotland Yard refused to comment on reports the man had passed through a security check at the castle by using the name of a senior detective currently heading a high profile investigation.
The man, from East London, was arrested in the public area near the Round Tower at 1615 BST on Monday, along with the woman, a Scotland Yard spokesman said.
The woman, from central London is being interviewed at a London police station.
"Police do not believe that at any point either of the two arrested people
breached the secure protected area of the castle, " said the police spokesman.
"Security matters are taken extremely seriously and as a matter of routine
the circumstances of these arrests will be reviewed as part of the police
Metropolitan Police Commissioner Sir John Stevens said the man was known to the police and officers needed to question him further before deciding whether to charge him.
A Buckingham Palace spokeswoman said the Windsor incident was a police
matter but the security adviser Brigadier Jeffrey Cook had been informed.
He was appointed security director in the wake of a major review of royal protection.
An overhaul of royal security was ordered last year after stand-up comedian Aaron Barschak gatecrashed Prince William's 21st birthday party at the castle.
He set off seven alarms and was visible on CCTV cameras but was not stopped by police.
Months later, royal security again hit the headlines after Daily Mirror journalist Ryan Parry was able to get a job as royal footman.
He said no rigorous security checks were made on his background before he gained close access to the royals.
Shadow home secretary David Davis said: "It beggars belief that within a
year, we have a third major breach of palace security".
"The home secretary must take responsibility for this and must now recognise that the case for a separate minister for homeland security is unarguable.
"If David Blunkett cannot protect our head of state, how can he be expected
to provide protection for the whole of the British public against terrorism