From an A-list falling-out to what ended an engagement: The biggest bombshells from new biography on Gwyneth Paltrow

A new biography about actress Gwyneth Paltrow is being released later this month and it has some bombshell revelations within its 448 pages.

Gwyneth: The Biography claims to know why the star broke off her engagement with Brad Pitt and the truth behind the end of her friendship with Madonna.

The juicy memoir is written by Amy Odell, who has previously published a biography about Anna Wintour, and is said to be the result of 220 interviews with "close current and former friends and colleagues".

READ MORE: Company launches investigation after Coldplay kiss cam drama

Its July 29 release date comes months ahead of her acting comeback, starring opposite Timothée Chalamet in Marty Supreme, slated for a December release.

Here are the biggest bombshells in the book that we know about so far:

What ended her engagement to Brad Pitt

They were the 'It' couple of '90s Hollywood.

After meeting during an audition for Legends of the Fall, the couple were then later cast as husband and wife on the movie Se7en, with their relationship blossoming while making the movie in late 1994 to early 1995.

The couple got engaged in December 1996 before splitting in June 1997 and neither star has ever said why.

According to the book, a makeup artist on the set of Paltrow's 1996 hit Emma claimed the actress was in tears about their relationship on multiple occasions.

It's alleged Pitt was jealous of his fiancée's success and Paltrow thought they were raised too differently.

"You really need to end this," make-up artist Kevyn Aucoin told Paltrow, according to an excerpt of the book published by People magazine.

It's also claimed the actress developed a crush on Hugh Grant while spending time in the UK making the Jane Austen film.

How she felt about him moving on with Jennifer Aniston

When Paltrow was asked by a reporter at the 2000 Toronto Film Festival, "how she felt about Brad Pitt marrying Jennifer Aniston" in July of that year, the actress' reaction was frosty.

"Are you really asking me this question?" she asked the journalist with her eyes becoming daggers before she added, "I can't comment on this kind of thing."

But in reality, she was very "sad" when she learned the news, according to an excerpt of the memoir from Us Weekly.

"In reality, Gwyneth confided to friends that she'd felt sad when she learned they were getting married. (She was also fond of telling them that Brad 'has terrible taste in women')," the excerpt reads.

She seemed to have kept this resentment for a long time, even bringing it up during a business meeting with Estee Lauder cosmetics heiress Aerin Lauder later that year.

"At one dinner, the two were talking about Brad Pitt. According to someone familiar with the discussion, Gwyneth allegedly told her, 'He's dumber than a sack of s–t,'" the book read

Madonna friendship fallout sparked by holiday dinner

The unlikely friends apparently hit it off after a potentially awkward introduction through Madonna's first husband, Sean Penn.

It's understood Paltrow's famous dad, director Bruce Paltrow, asked his friend - and Penn's dad - director and actor Leo Penn to get his son's wife to ask his daughter to stop smoking.

Madonna wrote Paltrow a letter, saying "Good girls live longer".

While it could have gone down the wrong way, the young actress and Material Girl became friends, the friendship becoming closer when they both lived in London when Madonna was married to Guy Ritchie and Paltrow to Chris Martin.

But according to the book, things turned sour after Madonna turned up at the island where Paltrow and Martin were holidaying.

"Madonna seemed to know that Gwyneth would be there, which Gwyneth seemed to find strange," Odell writes.

"Madonna then insisted Gwyneth and Martin join her for a big group dinner at a long table where Madonna went off on her daughter, Lourdes."

Odell claims the couple were "disgusted by the behaviour".

"I can't be around this woman any more ... she's awful," the Coldplay frontman allegedly said, to which Paltrow agreed and ended the friendship.

Her Oscars dress wasn't supposed to look like that

The actress won her first Oscar in 1999, at the age of 26, for her role in Shakespeare in Love wearing a pastel pink Ralph Lauren ballgown.

When the young nominee was working with the designer to come up with the concept for her dress, she wanted something inspired by Grace Kelly.

In the lead up to the big day, it's said Paltrow lost so much weight they had to keep refitting the dress and the actress kept requesting the neckline be made lower.

Lauren was said to be "so frustrated" by the process, and then on the night when he saw the dress still looked too big, he realised she didn't wear a corset created to go underneath it.

"The fabric was puckering, a little wrinkly, loose around her torso – as if she had instantly lost 10 pounds (4.5kgs)," Odell writes in the book.

"They realised she hadn't worn the inner detachable corset that came with the gown, which would have made it fit more snugly. Lauren was not happy about the fit."

Paltrow's focus on weight and health comes full-circle

According to the biography, in her high school year book, Paltrow listed her life's nightmare as "obesity".

Fast-forward decades later and Paltrow chose the 2001 movie Shallow Hal to get away from her serious movie persona after a raft of dramatic roles.

She controversially wore a fat suit to look like a 150kg woman in the Farrelly brothers comedy, centred on a man named Hal (played by Jack Black) who is hypnotised to see inner beauty.

"Gwyneth told friends that she felt like the film could bring attention to what would later be widely termed fat-shaming," Odell wrote.

During filming of the movie, it's understood Paltrow wore the full costume and went for a walk around Charlotte, North Carolina to see how people reacted to her.

"I got a real sense of what it would be like to be that overweight, and every pretty girl should be forced to do that," Paltrow said in an interview with Entertainment Tonight at the time of the movie's release.

The actress also commented on it in a September 2001 interview with W magazine.

"It was so sad; it was so disturbing. No one would make eye contact with me because I was obese," she said.

"I felt humiliated because people were really dismissive."

In the years since, Paltrow's focus on health, weight and well-being has come full circle in the success of Goop, which trades on her genuine interest in the wellness space.

Anyone needing support with eating disorders or body image issues can contact The Butterfly Foundation online or via their national helpline 1800 33 4673 (1800 ED HOPE).

For urgent support please contact Lifeline 13 11 14.

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