The music mogul displayed his affection for Hussainy yesterday during a break in filming at the Hammersmith Apollo in West London, according to The Mirror.
It is thought that Cowell was seen hugging his girlfriend and kissing her on the lips, marking a turning point for the previously private couple.
A source said: "This is serious. He is head over heels in love. The fact he's happy to show his affection in the most public manner says it all. She is everything Simon wants in a woman. They are blissfully happy."
Insiders have said that Hussainy has been accompanying Cowell at the Britain's Got Talent auditions for weeks.
The pair were first linked last summer. Earlier this week, reports indicated that Cowell was "ready to let everyone know" about the romance.