Bradley Cooper says a sequel to 'The Hangover' was already planned before the first film came out.

'The Hangover' - which tells the story of a stag do in Las Vegas - was a huge success when it was released in 2009 but producers of the outrageous comedy were so sure it would be a hit, they had already commissioned a follow-up.

Bradley - who is reprising his role as Phil - said: "We're looking forward to what you're watching right now, us back together again. Warner Brothers wanted to make a sequel before the first film came out. So it was planted in our mind before the first film was even released.

"It's been with us a long time, and we were hoping it would do well so they would follow through with that. But their interest started when they saw what was happening.

"Of course we couldn't anticipate it becoming what it was, but one thing I will say that we'd sit there in Caesar's Palace eating dinner after shooting and say "hey, this is pretty good, and this is kind of different."

'The Hangover Part II' - which also stars Ed Helms and Zach Galifianakis - is set in Thailand and Ed is hoping it will live up to expectations.

He told Collider.com: "Rather than just raise the expectation bar and say we're raising all the antes, I think there's just some fun creative surprises and twists on people's expectations."