The 29-year-old and his model girlfriend have parted ways due to their conflicting work schedules, according to The Sun.
"This has come as a surprise to everyone," a source said. "It was thought they'd be together for a really long time because they were so into each other. They always seemed so happy together. Matt was besotted with Daisy and she adored him.
"It's really sad that this has happened, but as they're both so in demand at the moment it was really difficult for them to find time together."
The insider further claimed that tensions had emerged between the couple over Smith's reluctance to get married.
"Whenever engagement and marriage was mentioned he was uncomfortable," they said. "He's still a young man and the thought of being tied down was a lot to digest."
Smith and Lowe have been dating since April 2010. They briefly split that May, but reconciled less than a month later.