Kate confesses she is besotted with American Beauty director

Exclusive by Shekhar Bhatia and Rick Hewitt

Kate Winslet is having a secret affair with Oscar-winning director Sam Mendes. The 26-year-old Titanic star has fallen madly in love with the American Beauty movie-maker. The actress who split from her husband Jim Threapleton in September, told a friend: "I love Sam - I just love him so much."

The Sunday Mirror can reveal that Mendes, 35, and Kate spent two nights together in an American hotel last weekend. They have also met in Britain on several occasions. During the summer they even used Mendes's mother's home near Oxford as a bolthole. Their busy careers tend to keep them apart but they are in constant contact by phone.

News of Kate's new love comes as a hammer blow to her estranged husband Jim, 27, who was stunned by her desire to end their three-year marriage quickly. It will also shock the couple's many celebrity friends - some of the biggest names in showbusiness - who have been unaware of the growing romance. Kate is hoping that the end of her marriage to assistant film director Jim is only "weeks away" after agreeing to a pay-off of under £500,000.

She has gone to great lengths to hide her relationship with Mendes from the world. Late last month Kate, who said no-one else was involved when she announced her divorce, insisted: "At the moment all the love I have is for my child." But last weekend Mendes, currently based in Los Angeles directing the new Spielberg movie, The Road To Perdition, jetted to be with her. Kate is filming in Austin, Texas, playing a reporter in the new Alan Parker film The Life of David Gale, a drama about anti-capital punishment activists. At the airport in Texas, Mendes was met by one of her assistants in the arrivals hall. He and a woman went back to a five-door jeep which had blacked-out windows. Suddenly, the rear passenger door opened and Mendes walked straight into Winslet's embrace. Winslet's aide then tactfully walked away from the vehicle to allow the director and the actress a few moments to greet each other in private.

Afterwards the couple were driven the 25 minutes to the Barton Creek Country Club Hotel, where Kate has been staying during her filming. Hotel staff watched as they were dropped off at a side entrance and Kate, wearing a long black coat, stayed close to her visitor as they walked along the hotel corridor towards the lift. They took the lift to the eighth floor, where Kate is staying, and entered her suite immediately. The actress, who is booked in under a false name with her baby daughter Mia, quickly placed a 'Privacy please' request on the door handle of the $700 a night suite, which has an adjoining bedroom.

The couple did not emerge until the next morning. First to appear was Mendes, wearing a blue and white checked shirt and jeans. He walked out on to the balcony of Kate's suite in mid morning. As Mendes walked back in, Kate, wearing a white dressing gown, came scurrying out and scooped up Mia, who had wandered out on to the balcony. Mendes spent the rest of the morning in the luxury suite before emerging after lunch in the hotel car park with the keys to the Jeep. He drove it to the side entrance and within minutes Kate emerged, holding her baby. Mia was strapped into the back seat and Kate climbed into the front passenger seat and Mendes drove away. They spent a happy afternoon out together. Back at the hotel Mendes parked the Jeep after dropping off Kate and Mia. They disappeared into the hotel where they spent the night. Mendes emerged the next morning to board a 7.45 am flight back to Los Angeles.

Kate had told her estranged husband she was dating Mendes a fortnight before the jetting off to Texas. She assured Jim the relationship had begun only after they ended their marriage. Jim has been telling friends he believes the divorce is now only "weeks away and maybe sooner". He was shocked by her divorce demands last September, believing the rift between them was temporary. Lawyers for both sides have been busy ironing out an agreement. The couple's £800,000 house by the Thames in Surrey has recently been sold. However, the bulk of the proceeds are going to Kate. Jim will receive a settlement from Kate. Despite her multi-million pound fortune, he is set to be given less than £500,000. He is going to meet Kate in America within the next fortnight - when he will confront her about her weekend tryst with Mia in tow.

Friends say Kate had told Jim and them she met Mendes last May to discuss business projects. "Jim never heard his name mentioned again until shortly before she left for America and told him about the relationship", a close friend said. He added: "Jim has never met Mendes and he doesn't want to. But he is concerned that Mendes doesn't become a part of his daughter' s life. Kate has assured him that Mendes will not be seeing Mia so he will be mortified that Mia was with Kate when she spent the weekend with Sam.

"This affair came as a great shock to Jim as he believed Mendes was simply going to direct Kate in a new project and that is how they got to know each other. Never for a minute did he think anything was going on between them. It's totally confused him. He still can't believe that his marriage is over, although he has had to accept it because Kate has been so determined to end it. He was shocked when things went downhill so quickly for him and Kate. He was saying that it seemed a bit sudden to bounce back into a relationship. He hasn't met anybody and has been busy working on a script - a drama.

"What he doesn't want is for his relationship with his daughter to be affected. He has been bending over backwards to allow Kate to take their child to America for nearly two months. He has been really cut up about it because he misses her every minute of every day. He is such a loving father. His world is built around Mia. What has been particularly difficult for him is that Mia is about to start walking for the first time and he is sad that he is missing out on that. It was tough enough being told his marriage was over - but knowing that somebody as famous and powerful as Sam Mendes is waiting in the wings is not giving him comfort. He gave up his own career to support Kate's career and he is trying to pick up the pieces now. He has been working hard on a script and he is determined to throw himself into his work to take his mind off his marriage breakdown. He didn't think things could get much worse but now they have."

Kate, whose career soared after the Hollywood blockbuster Titanic, was approached by Mendes in September to star opposite Nicole Kidman in a double bill at the Donmar Warehouse Theatre in Covent Garden. Mendes, the Donmar's artistic director, interviewed Kate twice about appearing in two classics - a Shakespeare and a Chekhov play - which are being stage next year to celebrate the theatre's 10th anniversary. A friend said last night: "At least if this project goes ahead they can spend lots of time together. They both have so much in common."