Hollywood actor Gary Oldman had to enrol in voice classes to perfect his speech for the film adaptation of classic British spy novel Tinker Tailor Soldier Spy - because the London-born native had lost his accent.

The star set up home in Los Angeles several years ago and had no idea how much his speech had changed since moving to the US until he signed on to play lead character George Smiley.

Gary Oldman as George Smiley in Tinker, Tailor, Soldier, Spy.

"The weird thing is, I had to do a bit of voice work to get my English accent back. I've not really lost my accent completely, but it is now a cross-pollination," he told W magazine.

The veteran actor's extra work paid off - his performance has landed him a Best Actor Oscar nomination.