According to OK! the couple spent the reception flirting and - said an unnamed insider - the Duchess was unhappy to catch them kissing when she went off to adjust her make-up.
Although Prince Harry allegedly pursued Miss Middleton for months, she didn't reciprocate for fear of offending her sister, says the magazine.
Eventually, they ended up with other people. Miss Middleton went out with financier Nico Jackson for three years, splitting up with him in October. The prince alternated for years between the model Cressida Bonas and Zimbabwean socialite Chelsy Davy. Not that love ever died between them, insists OK!.
"There's always been sexual tension between them, even when they were dating other people," a source told the magazine.
It claims that as her relationship with Mr Jackson was falling apart, Miss Middleton turned up one evening on the Prince's doorstep.
Pulling out all the stops for their first date, Prince Harry entertained Miss Middleton with candles, pasta carbonara and music by Adele.
Prince William is said to know about - and disapprove of - the romance but may be too late to stop them as they have a "real emotional connection".
The Palace has yet to comment, though it may not bother. OK! has a history of revealing celebrity couplings hotly denied by those involved.
- Daily Mail