Most of the park will close when it hosts 2012 events
Campaigners who want to stop Olympic equestrian events being staged in Greenwich Park have threatened to use a legal loophole from the 19th Century.
They said organisers for London's 2012 Games will be breaking a law from 1866 if they seal off parts of south east London's Blackheath.
A temporary 23,000-seat arena is planned to be built in the park during the Games.
Events including show jumping and dressage would be hosted at the venue.
Solicitor Lionel Lewis said: "There is an act which regulates the use of metropolitan commons. Blackheath was made a metropolitan common in 1871.
"The act specifically says no enclosures. What London 2012 wants to do is build a compound that's an enclosure and it's against the law."
Mr Lewis has put in an objection to the planners, who he said "can't give permission to something unlawful".
He said: "If they do go ahead with the plans, someone will have to put it in for judicial review."
London 2012's Tim Hadaway said: "We believe their interpretation is not correct."
"This is an area that is used already for many other events and there are times, for instance when the circus is here, when there would be areas that are not accessible to the public."
Residents group No to Greenwich Olympic Equestrian Events (Nogoe) has raised fears the venue will cause permanent damage to the Blackheath.
But the London Organising Committee for the Olympic Games said trees would not be cut down for the arena.