James Middleton - one of Britain's most eligible bachelors - has been reportedly dating the Newcastle-born blonde since meeting her in an exclusive London club two months ago.
The pair have been keeping their whirlwind romance under wraps, but are now said to have revealed to friends they are a couple.
One said: "It's still early days but James thinks she's great. They've been out together to a few places but they always choose discreet restaurants where they are less likely to get spotted."
James, 26, and Donna, 33, first met at a private members' club, 5 Hertford Street, in Mayfair, central London, in February.
Last month he accompanied Donna to New York socialite Heather Kerzner's 44th birthday party at Downtown restaurant in Mayfair, but there were no suggestions at the time that they were an item.
Donna, who counts Princess Eugenie among her friends, is believed to have met James's other sister Pippa in recent weeks.
James is said to have been keen to find a girlfriend since seeing Pippa, 29, settle down with 35-year-old stockbroker Nico Jackson.
James's first love was Australian Amy Bradshaw before he embarked on a long-distance relationship with model Emily Steel, who worked and lived in Japan. That relationship ended in January this year.
The businessman lives in Chelsea and runs cake baking company Nice Group.
Donna, who has a nine-year-old daughter called Freya, has recently been on holiday in the Bahamas, but returned home yesterday, writing on her Twitter page: "Home sweet home x".
In 2011 she wrote a monthly dating diary for website eHarmony and told followers: "We are all united in the search for the right mate."
The actress and presenter's most recent TV appearance was on Tom Daley's celebrity diving show Splash!
http://www.express.co.uk/news/showbiz/393797/James-Middleton-enjoying-romance-with-Donna-Air
