When the real Queen was unable to make the dreams of a dying 10-year-old come true by meeting him for a private audience, her showbiz 'deputy' stood in - and even made little Oliver Burton a knight of the realm!
The schoolboy, who has Down's syndrome, has recently been diagnosed with cancer for a third time. The number one request on his 'bucket list' had been to go to Buckingham Palace and meet HM, a request which the palace was unable to fulfill.
The charity National Children's Tumour Leukaemia Cancer Trust then contacted Dame Helen and the star arranged for the little boy and his family to meet her backstage at the Gielgud Theatre where she is currently playing Elizabeth.
Oliver's mum and dad, Catherine, 29, and James, 32, said Oliver was 'so delighted' and actually thought he was in the presence of the real queen!
Oliver's dad, James Browne said Helen stayed in character for the whole thing - and even brought along her corgis!
"She was really lovely. She did the whole thing - had a butler there, was dressed in costume and did it all properly for him," he said.
"She sat in Oliver's wheelchair and gave him her big chair. She had a glass of coke together and biscuits and little sandwiches and they even brought in her corgis from the show, Coco and Roco."
The chuffed family said the photos of their memorable day were so good that you had to do a double take as to whether it was actually the real queen or not!
"She knighted him and told everyone that they had to call him Sir Oliver. He had a brilliant day.
He took his British flag and got her to sign it and just waved and waved it all day," James told the Mail.
Oliver was just three when he was first diagnosed with leukaemia in 2006. His devastated family have twice been told their little boy has just hours to live, but each time brave Oliver has pulled through.
His latest relapse, however, cannot be cured, and his doctors have told the family he may have just weeks or months left to live. If he were to undergo more intensive treatment, medics have warned he would probably end up fighting for his life in hospital.
His parents have chosen to care for their son at home, and say they are determined to make his final weeks or months as happy as possible. Fund raising has enabled Oliver and his six-year-old brother Ethan, to have family days out and memorable trips, such as meeting 'the Queen'.
"Without these donations and this help we just never would have been able to have these amazing experiences as a family," Oliver's dad said.
"Before this we have never asked for help, just done things on our own. When you go through this you do lose friends because you are in hospital all the time or just at home caring for Oliver, but with all this we feel like we have friends again."