A specialist police team are monitoring more than 400 stalkers who could be a threat to the royal family, new figures reveal.
Officers and psychiatric nurses are visiting the most “high risk” fanatics and 11 have been arrested in the last three years.
The specially trained Fixated Threat Assessment Centre are also combing online sources to identity potential threats.
Referrals to FTAC involving those obsessed with members of the royal family have almost doubled in two years, from 73 in 2014 to 137 for the first eleven months of 2016.
In total 439 cases were sent to be monitored since 2013 including 20 deemed “high risk”.
Former royal protection officer Ken Wharfe, who retired in 1993, said: “From my experience it is important to treat these people very carefully because they are unpredictable.
“In the past we did not take the threat seriously from fixated individuals but during my time we began having officers in the crowds looking for known individuals like these.
It is a real problem as there figures demonstrate and it’s important that they are monitored closely because there is always potential for an incident.”
The FTAC, which also investigates potential threats to other VIPs, is made up of nine detectives, three nurses and a community support officer and is based in a town house opposite Buckingham Palace.
It is led by a psychiatrist and a psychologist. Officers believe the best way to stop stalkers getting too close is to monitor them in the community.
They reduce the risk of a violent incident by making sure stalkers have access to mental health services or placed in a secure unit.
Around 40% of Britain’s most dangerous stalkers focus on the Royals.
Many believe they are the true heir to the throne or that royals are either in love with them or possessed by demons.
Buckingham Palace receives 10,000 letters a year from people with mental illness.
Most are harmless but some contain threats. The Queen, Prince Charles and Princess Anne have all been targeted by stalkers and fanatics.
Forensic psychiatrist Dr Frank Farnham, who works with the team, said in 2014: “We have a significant number of people who believe they are the Queen or are in a love with Prince William.”
The figures were released by FTAC in response to a Freedom of Information request by the Daily Mirror.
It said 11 people have arrested in relation to threats made to the royal family since 2014 but provided no further details.
Dr David James, a Consultant Forensic Psychiatrist and lead clinician at FTAC, found evidence of fixation and psychosis in 23 attacks on the British royal family he studied between 1778 and 2003.
One of the most notorious cases was the attempted kidnapping of Princess Anne from her car in the Mall in 1974 by an armed man.
Ian Ball, 26, shot and wounded four people, including her chauffeur and bodyguard.
Ball remains in Broadmoor where he continues to be treated for schizophrenia.
Of those who threatened the royals, James more than a third stated they had “reached a position of last resort”.
Dr James said of the cases that occurred before he helped set up FTAC in 2007: “Information was coming in, but there was no system to assess it. And the attacks were potentially preventable.”
Royal raider Michael Fagan spent 10 minutes chatting to the Queen in her bedroom in a 1982 security breach
Fagan, 33, climbed over the Buckingham Palace walls before scaling a drainpipe. The Queen left the bedroom to call for help.