Seward, who has written more than a dozen royal biographies, including this year’s Prince Philip Revealed, was asked about the hit Netflix drama while appearing on a podcast.
While guesting on the Royally Obsessed podcast, the writer singled out Smith’s portrayal of the Duke of Edinburgh as one of the show’s weaker points.
“Prince Philip is so upright,” she said. “He has that military stance. So Matt Smith got it all wrong in The Crown.
“He was always bent over, and looks like he is being told off all the time. They got his look and mannerisms all wrong.”
Seward also identified a plotline in The Crown’s second season, in which Philip is suggested to have had an affair, as a point of inaccuracy.
She said: “There was a story about a ballerina that he was supposed to have had an affair with, but that was absolute rubbish.
“But The Crown is not a documentary,” she added. “You have to allow some poetic licence.”
The Crown’s fourth season is set to debut on Netflix on 15 November.
Alongside Olivia Colman as Queen Elizabeth and Tobias Menzies as a now-older Philip, the season will add Emma Corrin as Lady Diana Spencer, and Gillian Anderson as Conservative Prime Minister Margaret Thatcher.