The deal, which is still subject to the clearance by the European Commission as well as consent from the UK Pension Regulator, is expected to go through by the end of the year.
United Biscuits, which also owns crisp brands McCoy's and Hula Hoops, was founded in 1948 following the merger of two Scottish family businesses McVitie & Price and MacFarlane Lang.
In 2000, the company was bought by Finalrealm, a consortium of investors, including PAI Partners, Cinven and MidOcean and reverted to private limited company status.
United Biscuits is the number one company in the UK biscuit market, and is also the number two businesses in the biscuit markets in France and Belgium and joint number one in the Netherlands.
The company is number two in the UK bagged snacks market and UK cake market, and the leader in the UK branded nuts market. The firm employs more than 9,000 people, an estimated 7,500 of which work in the UK.
Malcolm Ritchie, chief executive of United Biscuits, said: "Blackstone has a successful track record of owning and developing companies such as UB and I believe the company will benefit from their involvement.
"I am delighted that PAI has decided to remain a shareholder and participate in the next phase of the company's development and growth. I would also like to thank both Cinven and MidOcean, our departing shareholders, for their support and valued advice over the past six years."
In July, US company Kraft Group paid £575m to acquire some United Biscuits brands including Ritz Crackers and Philadelphia cream cheese.
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