For well over a year the strong-willed daughter of theatre director Sir Peter Hall then refused to discuss the matter, point blank.
But it has now emerged that Miss Hall is in a relationship with Mr Mendes - although neither will specify exactly when things started.
Mr Mendes, who split from Miss Winslet early last year after a six-year marriage, has confirmed they are an item.
Miss Hall found herself cast as "the other woman" when stories about her friendship with Mr Mendes started circulating days after the Oscar-winning Hollywood couple announced their separation in March 2010.
At the time a source, said to be a friend of Miss Winslet, said the Titanic actress was "upset" that her husband had "grown close to Rebecca".
The friend said Miss Hall, whose father founded the Royal Shakespeare Company and whose mother is the American opera singer Maria Ewing, was "totally Sam's type - a thespian mix of brains and beauty".
The two got to know each other in 2006 while working on her debut film, Starter For 10, of which Mr Mendes was executive producer.
Miss Winslet, now 36, was said to have been unhappy that her husband subsequently chose to cast the younger actress in two subsequent theatre productions.
For her part Cambridge-educated Miss Hall, now 29, said she was no more than good friends with Mr Mendes.
Three months later, in her first interview since the rumours surfaced, she took the opportunity to dismiss them out of hand.
Asked if she was having a relationship with Mr Mendes, who won an Oscar in 2000 for directing his debut film, American Beauty, she firmly replied: "No."
"It's the weirdest thing and it's all a load of b*******," she added.
She insisted he was just "a good mate", saying: "The reality's just really boring."
Apart from denying the odd question, flung at her on the red carpet, she kept a dignified silence.
That was until earlier this month, when her answer appeared to have changed somewhat.
"I don't see the point in defending myself," she told a British newspaper.
Appearing to enjoy the ongoing speculation, she went on: "I would rather there was mystery and, frankly, confusion. It's easier that way.
"It's a really complicated thing, and I appreciate that if you're in the public eye everyone's going to want to know about your personal life. But it's my personal life! And it's precious!"
It has now emerged the two are seeing each other, after Mr Mendes was asked to confirm the matter to a Sunday newspaper.
His spokesman confirmed they were now dating.
The 46-year-old actor is currently in London directing Skyfall, the new James Bond film starring Daniel Craig.
A spokesman for Miss Hall was yesterday unavailable to comment.
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