Prime Minister David Cameron said as "The Iron Lady" opened in Britain Friday that the film showing former premier Margaret Thatcher's dementia should have been delayed until after her death.

The biopic shows Thatcher as a frail, sometimes confused old lady -- she is now 86 and is rarely seen in public -- looking back at her career with the ghost of her late husband Denis looking on.

Cameron, in his first comment on the film, said he had been impressed by Meryl Streep's portrayal of Britain's first woman prime minister.

But Cameron questioned whether it was right to make the film while Thatcher was still alive.

"It's a fantastic piece of acting by Meryl Streep but I just can't help wondering why do we have to have this film right now?" he told BBC radio.

"It is a film much more about ageing and elements of dementia rather than about an amazing prime minister, and my sort of sense was a great piece of acting, a really staggering piece of acting, but a film I wish they could have made another day."

Streep, who is tipped for an Oscar for her performance, has said she relished the "opportunity to play someone at the waning of her life... and that interested me too because there aren't very many films that pay attention to older ladies."