A no-fly zone has been imposed over the Wimbledon tennis championships, police chiefs revealed Monday, as they strengthened security at the All England Club ahead of the Olympics.
The air exclusion zone was last used in the two years following the September 11, 2001 attacks on the United States.
The zone stretches for 0.75 nautical miles (1.4 kilometres) around the complex in southwest London and extends 500 feet (150 metres) high, Superintendent Pete Dobson told reporters.
"You just don't want strange aircraft flying overhead if you can avoid it from a security point of view," he said, as the two-week annual championships got under way.
Dobson said security had been heightened due to the London 2012 Games. Wimbledon will host the Olympic tennis tournament.
"After this event finishes, there are 18 days before the start of the Olympics so I'm acutely aware that if somebody was to do something untoward here, it would have a knock-on effect for security at the Olympics," he said.
Police numbers have been boosted this year but Dobson would not divulge the figure for security reasons.
"We do make some tactical changes so we can defeat anyone who's trying to do a hostile reconnaissance," he said.
"The uniqueness of this event (Wimbledon) is that it provides you with a large window of opportunity. It's not a one-day event.
"So it's possible that somebody with untoward intent -- whether they're a terrorist group, a protest group, or an individual -- could look at what we're doing. We have to take that into account when we design our policing plan.
"The fact that the Olympics are here also means we've been able to benefit from other security providers."
He said some of the Wimbledon command team would reprise their roles for the Olympics, though the Games policing at the All England Club "will look totally different" as it will be part of a consistent policing plan across the Olympic sites.
Dobson said there were undercover plain clothes police officers deployed around the grounds.
There are also armed officers on site to protect visiting dignitaries, such as royals, government ministers and ambassadors.
Meanwhile police are on the lookout for stalkers, who are a mixture of British and foreign nationals.
"They are the usual suspects, individuals of concern to women's tennis around the world," Dobson said.
"Some of those are stalkers of players. Some of them have been coaches that have been dismissed.
"No players have approached the police or the club with specific threats," he added.