Solange Knowles’ lift attack on Jay-Z was reportedly because he had reneged on promises to boost her music career.

Footage was made public yesterday of Beyonce’s younger sister seemingly lashing out at her brother-in-law in a ferocious attack while in a lift at The Standard hotel in New York following last week’s Met Gala. While there was no audio with the video, a source claims the altercation stemmed from Solange’s frustration with the 44-year-old for going back on his word to help raise her profile.

“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,” the insider told the Mail Online. “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.”

While big sister Beyonce has sold 118 million records worldwide, Solange has struggled to make an impact on the charts. The 27-year-old has released two studio albums, with the second only peaking at number 9 on the Billboard charts in 2008.

“Solange's [third] album is coming out soon but there is no buzz about it, which is a bad sign within the industry,” the source continued. “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. 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!"