Historian David Starkey has dismissed the idea that the Queen is the country's greatest monarch.
Dr Starkey said Elizabeth II will be known for her "record of unimpeachable integrity and simply keeping going", but that her lack of power prevented her from being Britain's most important sovereign.
As the Queen prepares to pass the record of her ancestor Queen Victoria to become the nation's longest reigning monarch in history on September 9, the historian told the Press Association: "It just means she's lived a long time. It's no more than that.
"Clearly women generally live longer than men. Most of the men of the House of Windsor killed themselves by smoking. I imagine the Queen doesn't smoke in private. She's following in her mother's footsteps by living a long time.
"Long reigns are interesting. It looked, in the Nineties, as if the reign was going to end on a sour note. I don't think there's any sign of that now."
Dr Starkey said the Queen encountered problems following her annus horribilis in 1992 and the reaction to Diana, Princess of Wales's death in 1997, but her reign has been rescued by two factors.
"Two things have happened to save her," the historian suggested. "The first is the contempt for the political structure and politicians. At the time of Diana's death, (Tony) Blair and (Alastair) Campbell extended a helping hand to Buckingham Palace. Nowadays it would be the kiss of death.
"The second thing, of course, are considerable expressed doubts about her heir (the Prince of Wales) - although I don't share them."
Asked whether passing Queen Victoria's record would elevate the Queen to the position of the country's greatest monarch, Dr Starkey responded: "What a ludicrous idea. The great monarchs are the ones who exercise power. The last monarch to exercise a vestige of really significant power was George V.
"The Queen is important for having established a record of unimpeachable integrity and simply keeping going. She's done that within her own terms."
He added: "I think she is immensely respected, but clearly she is not the nation's cuddly-but-a-bit-tipsy grandmother, like her mother.
"She has played the role with a certain dignity. In this touchy-feely age she has performed I think - with a certain degree - in a slightly old-fashioned way."
Dr Starkey highlighted the Queen's accession as one of the key moments of her reign.
"The nation had not been told how ill her father was and that sense of extraordinary vulnerability of this young woman and her astonishing composure - she never showed any diffidence about the role," he said.
"There is no doubt whatsoever that the role, in her understanding of it, sat upon her quite naturally."
The outspoken historian, who was once branded the rudest man in Britain, has attracted controversy over his views in the past, recently comparing the Scottish National Party to the Nazis.