Waitrose is set to strengthen its ties with the Prince of Wales after revealing plans to open a store on his estate in Cornwall.

The proposal which comes months after Waitrose signed an exclusive deal to licence the Prince's Duchy Originals brand has outraged local green and business groups, who are up in arms at the idea of a supermarket being built on Duchy of Cornwall land at Union Hill in Truro.

Waitrose claims the store, which would be its second in Cornwall, would showcase the best of Cornish produce. "In addition to bringing the Waitrose offer to Truro for the first time, it will enable us to increase the number of existing Cornish producers that already supply our stores," said a spokesman.

But the Cornish Guild of Smallholders doubts a new store would actually benefit Cornish producers.

"To a large concern like Waitrose, local food would just mean 'south west'," claimed chairwoman Joy Cheesman. "When they opened their Saltash store, they had no clue where local food was from. Any local produce that came in would have to bear the 'Made in Cornwall' logo, otherwise I wouldn't believe a word of it."

The idea that the new store would help Cornish producers was "pie in the sky", added Tony Hilton, retired chair of the Campaign to Protect Rural England, Cornwall.

"Like all supermarkets, if someone can come in at tuppence cheaper, they'll buy," he said. "I would discount that straightaway."

Writing on his blog, Truro and Falmouth Green Party candidate Ian Wright claimed that new supermarkets tended to have the effect of killing local businesses and reducing consumer choice.

"Building on this site will cause significant ongoing traffic congestion. To build on greenfield land when we already have more than enough supermarkets in Truro is wrong."

Responding to the concerns, Waitrose stressed that a park and ride scheme would be included in the proposal. "It is in the best interests of all concerned to ensure that there is ­convenient access, otherwise it won't achieve what we want it to," said the spokesman.

He also reaffirmed Waitrose's local sourcing policy, stating: "Waitrose is committed to supporting local producers throughout the UK, not only because of our sourcing policy but because our customers want food produced on their own doorstep."

The plans will be put to the local community at a meeting in June and, if they go ahead, Waitrose hopes to open the store in 2013. It would be the third supermarket to open on Duchy-owned land, adding to an existing Tesco in Dorchester and a Budgens in Poundbury.

And a source close to Duchy Originals said the plans were unconnected to the licensing deal.

"Duchy of Cornwall is a straightforward commercial venture and could just as easily have made a deal with B&Q."