The 61-year-old denied that she snubbed the Queen’s invitation to Buckingham Palace.

"I am mortified," Dame Helen is quoted by the New York Post as saying.

She said she was working on National Treasure in the Black Hills of South Dakota with Hollywood star Nicolas Cage.

"I just could not shut all that down to take three days off to go to Buckingham Palace," she said.

Dame Helen said in a statement: "I was honoured to be invited to dinner at the Palace. This was a gracious gesture and very appreciated by me. It was therefore hard to have to decline.

"I was contracted on that date to be working in South Dakota, in a situation which was impossible to change. I would have made every effort to attend if it had been humanly possible. I explained this to the Palace officials, and I believe they understood.

"I would never have the hubris or the rudeness to insult anyone who had the kindness to invite me to dinner."

Dame Helen to publish memoirs - April 20 2007
Dame Helen Mirren is set to publish her memoirs later this year.

The actress - who scooped an Oscar for her dazzling portrayal of the monarch in The Queen - will release Snapshots From My Life this autumn.

The illustrated book begins with pictures of the star’s Russian ancestors, which were brought to the UK by her grandfather, and charts her childhood and her years on stage.

It continues through her later career, which has seen her win countless fans as Prime Suspect’s Detective Jane Tennison and then on the silver screen in films such as Calendar Girls and Elizabeth I.

It will also give fans an insight into the 61-year-old life with her husband, director Taylor Hackford, and her experiences of Hollywood and at awards ceremonies.

Snapshots From My Life will be designed and published by Weidenfeld & Nicolson Illustrated. The hardback, priced £20, is due to hit the shelves in September this year.

Helen buys her own castle? - April 4 2007
Dame Helen Mirren’s royal role appears to have rubbed off on her - as she’s now reportedly splashing out on a castle of her own.

The 61-year-old, who scooped a Best Actress Oscar this year for her convincing turn in The Queen, is apparently finalising the purchase of a southern Italian castle in the Tiggiano region.

"Helen was really enthusiastic about the property," a source told US magazine Us Weekly.

But, despite reportedly paying in the region of £700,000 for the estate, it doesn’t look like Helen and her film director husband Taylor Hackford will be able to move into the 500-year-old property any time soon.

Although the castle, called Masseria Matine, comes complete with a moat and a dungeon, it currently has no running water or electricity and may need an estimated £550,000 worth of renovations before the glamorous pair can call it home.

"It’ll need a lot of work," the town’s mayor, Donato Martella, who toured the five-acre property with Helen on March 31, told the magazine. "But Helen has big plans for it."

Ovation as Helen brings Oscar home - Feb 28 2007
Dame Helen Mirren was greeted with a standing ovation by fellow air passengers as she brought her Oscar back to Britain. See photos of The Oscars

The star spent about 20 minutes posing for photographs and signing autographs in the baggage hall of Heathrow airport after flying back from Los Angeles.

Dressed casually in a white cotton blouse and looking fresh after her flight, she described it as an "incredible welcome".