The sixth instalment in the hugely successful Harry Potter film franchise will now open in July 2009, eight months later than originally planned.

According to Warner Bros, the decision was made to ensure the studio has a major "tent pole" release next summer.

Studio president Alan Horn also cited the recent Hollywood writers' strike and the effect it had on movies that were to have opened around that time.

The BBC News website spoke to industry experts about some of the other factors which may have influenced Warners' decision.