EXCLUSIVE: 'Jealous' Solange attacked Jay-Z in elevator because he 'keeps breaking promise to boost her career as she's tired of living in Beyonce's shadow'

Beyoncé's sister Solange lashed out at Jay Z because her multi-millionaire brother-in-law has reneged on repeated promises to help boost her music career, according to sources.

The 27-year-old singer - who is about to release her third album - was seen launching into an astonishing attack on music mogul Jay-Z at an after-party for the Met Gala.

And a family friend today revealed the simmering tensions that has led to the family feud, telling MailOnline: 'Solange is so angry because Jay-Z promised to give her more help with her career and when she brought it up again the night of the gala, he told her he just didn’t have the time to work with her in the studio.

'He hasn’t kept his promise to help her. Jay keeps telling her to give him some time and he will sit down and write with her and then get her into the studio but he hasn't done it yet.'

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Annoyed: Solange seemed upset by something as she posed for pictures with her sister at the Standard Hotel's Boom Boom room following the Met Gala last week

Crazy night: Kim Kardashian's pal Rachel Roy (left) 'publicly argued with Solange' (right) before singer attacked Jay Z in an elevator at Met afterparty, according to UsWeekly

The source added: 'Solange's album is coming out soon but there is no buzz about it, which is a bad sign within the industry.

'Solange wants a major career like her sister and she's tired of living in her shadow. She's jealous.

'Jay-Z promised he would help take her to the next level but he never found the time to keep his word.'

Speaking about the incident in an elevator at The Standard hotel in Manhattan, the source claimed: 'Enough was enough for Solange! She flipped out and screamed at Jay-Z that if he didn't want to help just say so and pulling her chain!"

To add insult to injury,  UsWeekly has reported that Solange first lost her temper with fashion designer Rachel Roy, a close friend of Kim Kardashian.

The publication claims that Solange 'yelled' at Rachel - who recently attended Kim's baby shower - on the night of the Met Gala after-party.

A source tells UsWeekly that Beyonce had got in the middle of an argument between the pair, although it is unclear what it had stemmed from.

It was also reported that Solange was seen arguing with a number of other people at the bash.

Rachel is considered to be one of Kim Kardashian's closest confidantes and was even invited to Kanye's proposal to Kim last year.

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Jay Z PHYSICALLY ATTACKED by Beyonce's Sister Solange - Watch More Celebrity Videos or Subscribe

On damning video footage, Solange can be clearly be seen kicking and swinging violently at Jay-Z, the father of her two-year-old niece, Blue Ivy, after entering an elevator with her sister and her husband.

The clip shows a man who appears to be a bodyguard holding her back, but Solange still manages to rain blows on Jay-Z several times.

The bodyguard can also be seen pushing the emergency stop button in the elevator at the 12th floor, no doubt to stop the fight spilling out in front of photographers.