The US firm said it was planning a 100 million expansion of the studio, used for all eight Harry Potter films and other recent blockbusters such as The Dark Knight and Inception.
The expansion programme, due for completion in 2012, will mean the studios in Watford will represent around a third of Britain's total film stage space.
It is the first time a Hollywood studio has purchased its own permanent production facility in the UK. Warner said the expansion would create about 1,500 jobs at the studio and a further 300 for building and specialist contractors.
Barry Meyer, its chairman and chief executive, said: "For 86 years, Warner Bros has been intrinsically involved in film production in the UK.
"Our purchase of Warner Bros Studios, Leavesden, and our multi-million-pound investment in creating a state-of-the-art, permanent UK film production base further demonstrates our long-term commitment to, and confidence in, the skills and creativity of the UK film industry."
The 170-acre site is currently being used to film the second Sherlock Holmes film, directed by Guy Ritchie. After the expansion the Leavesden studio will be capable of hosting two major films as well as advertising and television shoots.