Dame Helen Mirren admits portraying Queen Elizabeth on stage will be a ''challenge''.

The 67-year-old actress - who has previously played the monarch in hit movie 'The Queen' - is to star in 'The Audience' in London's West End next year and admits the role will be tough as she has to portray the royal from when she took the throne aged 25 right up until the modern day.

She said: ''That'll be a challenge.''

However, the actress has given the role some thought and thinks the monarch's ''unbelievable consistency in everything'' will help with the transition, and she is also studying the changes in her speech patterns.

She explained to the Daily Mail newspaper: ''Her voice has changed and I can use that - she had a terribly posh voice when she was young.

''But now even the queen, while she isn't quite dropping the ends of her lines - though her grandsons do - there's a tiny bit of estuary creeping in there.

''I can use all that to signify the age range and we'll come up with other things.''

Helen - who also played Queen Elizabeth I in TV series 'Elizabeth I' and Queen Charlotte in 'The Madness of King George' - admitted she was initially apprehensive about taking on the role.

She said: ''I certainly don't want to be known as the actress who played the queen, so there was a slight sense of apprehension about that element.

''I was thinking, 'You really shouldn't be doing this Helen, I don't think it's right.' And the thought of doing a play - you can't go anywhere for six months.''

'The Audience' - which examines the relationship between the queen and her Prime Ministers - begins previews on February 15, officially opening on March 5 with a run scheduled until June 15.