Is this why Solange attacked Jay Z?

Yesterday's incident wasn't the first time Solange had attacked Jay Z, it has been claimed. Photo/AP

Solange Knowles reportedly lashed out at Jay Z because he has failed to help boost her career.

The singer, 27, physically attacked her brother-in-law in an elevator at the Met Gala after-party in New York City on Monday night because the rapper reneged on repeated promises to work with her in the studio.

A source told the Daily Mail: "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."

Surveillance footage emerged yesterday of Beyoncé's younger sibling hurtling towards Jay - real name Shawn Carter - in a lift in The Standard Hotel, while the XO hitmaker watched on in the background.

Prior to the drama, Beyoncé, 32, and Solange were seen dancing up a storm on the dancefloor with supermodel Naomi Campbell, while Jay, 44 - who has two-year-old daughter Blue Ivy with his spouse - "hung back at one of the fireplace areas".

Despite the controversial video, Beyonce and her husband were spotted last night at a basketball game between the Brooklyn Nets and the Miami Heat at the Barclays Centre in New York and appeared relaxed and carefree.

Meanwhile, it has been claimed this isn't the first time Solange has attacked Jay.

A source told Us Weekly magazine: "Solange has flipped her lid like that before.

"She is genuinely a sweet and nice person but she also keeps s*** real. She can take a small thing and it triggers her.

"This is not the first time this has happened. That's why Bey and Jay were so calm. They are used to this from Solange. It has definitely happened before."

At the end of the evening, Solange and Beyonce left in one car, while Jay left in another, and insiders say it was typical of the Crazy in Love singer's "calm" nature to try and keep the peace.

A source said: "It is a testament to Beyonce's calm nature how she kept back and played the peacekeeper here. She is so calm and always the peacekeeper."

However, an onlooker at the Met Gala claimed the singer didn't seem like herself throughout the evening.

The source said: "She wasn't smiling, was hardly with Jay, she was off with Solange the whole time."

Another report suggests it wasn't the only fight Solange got into that evening.

A source claims Solange yelled at numerous people, including fashion designer Rachel Roy, who was once married to Jay's former friend and business partner Damon Dash.

- Bang! Showbiz