Shockingly, the writer also revealed that the Duke of Cambridge's circle made snobbish remarks about his girlfriend, despite her sense of royal decorum.
"There are plenty of snobbish insults from people who, despite Middleton’s obvious attributes, believe that the monarchy needs to stop marrying 'out' of its class," she wrote.
"One member of the young polo-playing set in the county of Gloucestershire – known in the British media as the 'Glossy Posse' – noted, 'Her parents have a tarmac drive, for God’s sake.'"
Ward also noted that William's sister-in-law, Pippa Middleton was also slammed for desiring to "marry well."
In his 2011 book William and Catherine biographer Andrew Morton also detailed how Prince William's future wife felt at the time.
"Kate would have sensed reservations about her in certain quarters," he wrote.
"While she was seen as a ’pretty and sensible’ girl, there were those in royal and aristocratic circles who believed that the Middletons were too middle-of-the-road for the House of Windsor."
A Scottish nobleman allegedly told Morton: "It was felt that she wasn’t from the right stock.
"William was expected to find a suitable bride among the aristocracy or European royalty – one of his own kind.
"Kate was treated abominably behind her back by some who should have shown better manners."
WATCH: Prince William and Kate Middleton spotted on a budget flight amid Meghan and Harry's private jet outrage
The royal family visited Balmoral this month - over the weekend William and Kate joined Queen Elizabeth, Prince Charles, Camilla Parker-Bowles and several other royal family members at a church service in Aberdeenshire.
Kate and William were also spotted with their children on a budget flight to join the Royal Family on their Scottish retreat and then returned home on a similarly priced airline.
The Duke and Duchess of Sussex - who are expected to announce their second pregnancy soon - are famously eco-friendly and as a result, the royal couple were slammed for their hypocrisy.