Britain's Duchess Catherine has an estimated 220 stalkers.
A specialist police team and psychiatric nurses are visiting the most "high risk" fans of the 33-year-old royal - who has 19-month-old son Prince George with husband Prince William - ahead of her giving birth to her second child next month.
And the Fixated Threat Assessment Centre (FTAC) are also searching for potential threats online, because it is believed the danger from stalkers increases in the lead up to significant events such as birth and marriage.
Dai Davies, former head of royal protection for the Metropolitan Police, told the Daily Mirror newspaper: "They give rise to a greater likelihood of an incident. If you look back through history the greatest threat to the Royal family apart from the IRA has been fixated individuals.
"Prevention is one of the key areas because in about 70% of cases where there have been attacks, the fixated person has indicated before the incident that they are going to do something."
The FTAC - which comprises nine detectives, three nurses and a community support officer - is led by a psychiatrist and a psychologist and investigates potential threats to VIPs. They believe the best way to stop stalkers getting close is to monitor them in the community and reduce the risk of a violent incident by making sure stalkers have access to mental health services.
Buckingham Palace receives 10,000 letters a year from people with mental illness, most of which are harmless.
Forensic psychiatrist Dr Frank Farnham, who works with the team, said last year: "We have a significant number of people who believe they are the Queen or are in a love with Prince William."