According to The Sun, the relationship came to an end because Thompson wanted to start a family, while Donnelly wants to continue living a bachelor lifestyle.
The Britain's Got Talent presenter said that he was "upset and disappointed" about the split, adding: "I honestly don't want to go into why we split. I'll always have very fond memories."
A friend of the couple said: "Most people in a relationship that long would have moved in together - but they didn't.
"There were no big rows, it just reached the stage where they had to decide where it was going. They love each other dearly but couldn't see it turning into a marriage. Georgie is 34 this year and, like most women at that age, is thinking about marriage and kids.
"Dec just isn't thinking like that. He's a very private person and likes living on his own."
The couple had been dating since January 2009.