Her gaze was coquettish, the lingering embrace too affectionate for comfort. When Hugh Grant cuddled up co-star Drew Barrymore and fixed her with his famous blue eyes, Jemima Khan could do nothing but watch stiffly from the sidelines, a look of exasperation etched on her face.

Was that the final nail in the coffin for one of the most talked-about relationships in the country? The final embarrassment that this weekend prompted the 33-year-old heiress to tell the world her three-year partnership with Grant is over?

Certainly, 31-year-old Barrymore, Grant's co-star in the new romantic comedy Music And Lyrics, is newly single, having split from rock star Fabrizio Moretti.

But this display of affection at their movie's New York premiere, which Jemima said made her look like a 'jilted woman', is not the whole truth behind her break-up from Hugh.

The real cause for the split was her unbearable suspicion that Grant is conducting another, altogether more secret, romance with a highly-placed figure in the film world.

Jemima has become convinced her 46-year-old boyfriend is involved with a glamorous young Cambridge-educated film executive from Warner Bros. Her suspicions were aroused by a series of late-night calls from an upper-class female with a 'plummy voice'.

Prolonged absences when Hugh would leave the house and turn off his phone only fuelled Jemima's insecurities, friends say.

"Jemima thought that Hugh was using Drew to deflect her from another love interest,' a source revealed last night.

"Sure, Jemima was convinced that Hugh had a soft spot for Drew. He would wind Jemima up constantly by talking about what a great sense of humour Drew has, how much she made him laugh and how she was one of the most engaging women he's met in a long time.

"It really annoyed Jemima, especially as she put up with Hugh being away so much to make the movie.

"But the truth is Jemima was far more concerned about a young woman at Warner's who kept calling Hugh, not a cheap publicity stunt with Drew. She became convinced something was going on.

"She found out the woman was rather posh and had been educated at Cheltenham Ladies College and Cambridge University. Hugh seemed quite enamoured by their conversations and told Jemima the woman was very senior in the studio.

"Hugh was always disappearing for meetings and whenever he was on the phone to this woman, Jemima could hear her plummy laughter. She would always call Hugh on his mobile, but Hugh would tell Jemima the woman was calling to discuss the movie. Then he'd switch his phone off. Jemima has been very upset about it. She's barely eaten for weeks.

"The other day he compared her to Victoria Beckham, which Jemima said was the ultimate insult. The relationship has not been good for a while. Jemima cares very deeply for Hugh, but she feels he's an incorrigible flirt and a bachelor who won't change his ways.

"The premiere of Music and Lyrics in New York last Monday was the turning point. Jemima realised there was no way she was going to be able to tame Hugh and that the relationship was over."

SPOKESMAN for the couple would make no comment on their relationship last night, saying: "Neither party is prepared to make a statement. This is a private matter."

But the four-year romance has been far from easy, with friends saying Hugh is 'difficult' and 'uncomfortable in his own skin' while Jemima can be 'tricky'.

After the breakdown of her 11-year marriage to cricketer-turned-politician Imran Khan, with who she has two sons, it was only a matter of weeks before Jemima started stepping out with Grant. But scarred from her divorce, she insisted she was in no hurry to marry again.

"The marriage issue is a bit of a misconception,' explains a close friend of Jemima's. "Despite reports they were engaged, they weren't. It's really not the case that Jemima wanted to get Hugh down the aisle. She is enjoying her new-found freedom and has no intention to marry again in a hurry.

"She did it for Imran, and lived a life in Pakistan that she probably wouldn't have chosen for herself. She submitted herself to Imran's lifestyle for many years for the sake of her children.

"She won't do that again in a hurry, especially not for an actor who's not that comfortable in his own skin and seems to have a lot of his own issues to deal with before he can start to think of committing to someone else.

"Jemima has everything in the world she could want - beautiful children, more money than she knows what to do with and fame in her own right.

"What she wanted from Hugh wasn't necessarily marriage, but emotional commitment. The relationship wasn't working because there was no empathy. He didn't provide her with the love and emotional support she needs."

In truth, it is likely that the pressure for the couple to formalise their relationship stemmed from Jemima's traditional upbringing.

Privately, Jemima's mother Lady Annabel Goldsmith has made no secret of her wish for Jemima to fill her £18 million Chelsea mansion with more grandchildren.

While Lady Annabel welcomed Grant into the Goldsmith fold, sources says she never enjoyed the sort of friendship she shared with Imran. And when Jemima had concerns about her relationship with Grant over Christmas, she turned to her mother.

"Jemima is incredibly close to her family, especially her mother, whose counsel she trusts implicitly,' says a friend. "Jemima said that she was lonely in the relationship and wasn't getting anything out of it. She would complain that Hugh was often away filming, and she said at times she was as good as single.

"Her mother may have put up with infidelity from her husband but she still believes in marriage. She asked Jemima whether she thought there was a future for them, and Jemima couldn't honestly answer. That made Jemima start thinking about things. It was better when the children were there to break things up, but Jemima complained that she was bored on a lot of their holidays.

"She's young and fun and loves socialising and going out but Hugh would just want to either play golf or stay in. Jemima would often be by herself when they were on holiday and would complain that she was becoming a golf widow.

"On one of their holidays to Grant's place in Royal Westmoreland in St James, Barbados, Jemima called home saying she was bored to tears."

The couple's relationship first hit the rocks just a year into their courtship. At a summer charity ball in 2005, Jemima confided tearfully to a friend that Grant was never around and that she was fed up with going to show business parties alone. She had several teary telephone conversations with Hugh during the evening.

A friend said: "Bizarrely this time round it's not about the lack of time they've had together, it really is a case of the relationship having run its course. Jemima's insecurities about Hugh have just compounded things. He doesn't help himself and is a terrible flirt, publicly and privately."

And Hugh's ex-girlfriend Elizabeth Hurley has been the permanent shadow over their relationship.

The rivalry between Hurley and Jemima has been well documented and privately the two women are said to despise each other. So steeped is the rivalry that Jemima banned Grant from seeing his ex, and as The Mail on Sunday reported last month, ensured Hugh would not be able to make her March wedding by booking a holiday abroad at the same time - and making it quite clear to Hugh that he was expected to join her.

While Grant still speaks to Elizabeth, he is said to resent the fact that he feels guilty when he does.

"Hugh and Elizabeth barely see each other any more, and that's only the case because of Jemima,' says a friend of the model.

"Jemima thinks Elizabeth lacks class and cannot understand why she still needs Hugh so much in her life when she's getting married now and should have moved on."

But while Elizabeth is said to be scathing of Jemima and considers her to be boring, friends insist she is an ambitious and focused woman.

"When she was growing up Jemima was the tomboy of the family and she has always been incredibly feisty,' said one source. "She hates the wallflower image others paint of her. They clearly don't know the Jemima who broke Lord Freddie Windsor's arm when they were play fighting when they were younger.

"Jemima can be hard as nails and incredibly spirited. If there's any chance of her and Hugh getting back together, it will have to be Hugh who does the running. Jemima has stuck with the relationship and worked at it, and ultimately she ended it.

"She is a woman of great pride and the one thing she would not allow is for Grant to make her look a fool.

"She's incredibly beautiful, rich and desirable and if Grant doesn't want to make things work, Jemima knows there are many eligible men out there who would jump at the chance."