Ozzy Osbourne has vowed to get clean and sober for his daughter Kelly's wedding.

The 64-year-old rocker - who recently admitted he relapsed and has been addicted to prescription drugs for the past 18 months - has promised the 'Fashion Police' co-host that he will get his life back on track before she ties the knot with vegetarian chef Matthew Mosshart so he can walk her down the aisle.

A source close to the 28-year-old star told gossip website RadarOnline.com: ''Kelly's using a little blackmail to help her father get clean sooner rather than later. She knows he has to want it, but she thinks a little tough-love encouragement can't hurt.

''Kelly went right for daddy's heart - she made it clear that she needed him to be present when he walked his little girl down the aisle. Ozzy would never ruin her big day, so he promised Kelly that he would get it together way before her wedding to Matthew.

''Ozzy knows better than anyone that he can't sober up for anyone except himself.

''But he says he's motivated by the fact that Sharon left him and now he's promised Kelly that he would walk her down the aisle only if he's clean. Kelly even threatened that if he wasn't coherent by the time of the wedding that she would have her brother Jack give her away.''

Kelly and Matthew, who met at Kate Moss's wedding to Jamie Hince in July 2011, reportedly got engaged before Christmas.

Kelly's mother Sharon, 60, recently admitted that she ''didn't realise the extent'' of her husband's problems with prescription drugs.

However, the mother-of-three insisted she has no plans to walk away from their marriage.

She said: ''We're not getting divorced. However, am I happy? No. Am I upset? Yes I am. I'm devastated right now.

''It's a disease that not only hurts the person that has the disease but it hurts the family. It hurts people who love you. We're dealing with it. We've dealt with worse. We're dealing with it and this too shall pass.''