WAITROSE is understood to be finalising an ambitious investment programme that could result in the upmarket supermarket chain investing about £800m in store openings.

It is thought the company is putting the finishing touches to its spending plans this month before they are ratified by its parent, the John Lewis Partnership.

The cash would be used to help Waitrose open 50 new stores over the next five years. The company has secured 10 new sites for next year already. Waitrose has also decided to roll out its new-look store format – recently tested at its branch in London’s Marylebone High Street - across the rest of its 183 stores. A new 25,000 sq ft store, which insiders describe as “Marylebone on steroids” will also open shortly in Finchley Road in the capital. Next month the company will also launch a food hall at the John Lewis department store in Oxford Street.

The launch, on October 3, will represent an important milestone for the John Lewis Partnership because it will be the first time that it has brought together the Waitrose and John Lewis brands in the same place. Branded the “Foodhall from Waitrose at John Lewis” it promises to be a foodie’s heaven.

If successful, the format could be rolled out to other John Lewis stores in city centres around the country, such as the Cardiff branch, which will open in 2009. The proposals all form part of a grand plan to double sales at Waitrose to £8 billion over the next decade.

The news came as it emerged that Waitrose plans to write to the Office of Fair Trading after the watchdog claimed last week that it had found evidence that the big four supermarket chains were involved in a cartel to fix the prices of milk, butter and cheese during 2002 and 2003.

The four biggest supermarkets – Tesco, Asda, Wm Morrison and J Sainsbury – have all denied any wrongdoing. Waitrose plans to write to the OFT pointing out that it paid farmers a premium price for milk during that period. Waitrose, writing as an independent party, will seek to establish there is no evidence its larger rivals were colluding more widely on price to squeeze out smaller players such as itself.

The big four were this weekend digesting the contents of a 600-page document detailing the allegations.

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