The '99 Problems' hitmaker had been celebrating his headline set at the BBC Radio 1 Hackney Weekend music event backstage on Saturday (23.06.12) with friends and family including Solange, his wife Beyonce Knowles, Rihanna and M.I.A. but he made sure his sister-in-law's special day didn't go unnoticed as soon as the clock struck midnight.
He presented her with cake and champagne and led a chorus of 'Happy Birthday', much to Solange's shock.
A source told the Daily Mirror newspaper: ''It was really sweet. Solange hadn't even noticed that it had turned 12am when Jay got everybody singing together. She was really touched.''
Earlier in the day, Solange and her sibling Beyonce had enjoyed a relaxing day at the festival before watching Jay-Z take to the stage.
Alongside a picture of herself and her sister with beaming smiles, Solange tweeted: ''We. Are. Family.''
The birthday celebrations will have come as light relief following an incident earlier in the evening when Jay-Z's bodyguards were allegedly involved in a ''ruckus'' at BBC Radio 1's Hackney Weekend after someone insulted Beyonce.
The rapper's entourage reportedly ''piled in'' on a man after he upset the rapper's wife backstage at the London music event.
A source said ''Someone in the entourage said something to Beyonce to really upset her.
''Jay-Z's bouncers moved in and there was a massive ruckus. Things really kicked off as his heavies all piled in on the guy.''
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