No value was revealed for the sale of the KP Snacks division, the UK's second-largest snack maker, but reports suggested the figure could be between £400 million and £500m.
Around 1500 people work for Middlesex-based KP Snacks at factories in Ashby de la Zouch in Leicestershire, Rotherham, and Billingham and Consett in County Durham.
Subject to completion next year, they will join the UK savoury snacks business of Intersnack, which makes Pom-Bear crisps and Penn State pretzels.
United, which is owned by private equity firms Blackstone and PAI Partners, will retain the baked bagged snacks brands manufactured in its biscuit factories, including Mini Cheddars and Twiglets.
United was bought by the private equity firms for £1.6 billion in 2006 and has been the subject of sale speculation in the last couple of years.
It dates back to 1948, when it was formed from the merger of two Scottish family businesses – McVitie & Price and MacFarlane Lang.
Intersnack is on track for record sales of £1.4bn this year, including £80m from the UK.
Executive chairman Maarten Leerdam said the company appreciated the "significance of acquiring a part of British heritage".
He added: "Intersnack is a strong believer in the power of iconic, local hero brands, and we aim to leverage these strengths for further expansion."