Parking charges introduced last month by Waitrose could be illegal as campaigners claim they directly break promises given on behalf of the food giant to a public inquiry into the closing of a footpath across the Monmouth site.

Stephen Clarke MBE, who presented evidence to the public inquiry in 1995, said: "It is quite disgraceful that the supermarket should try to renege on its clear undertaking to provide free parking for two hours for anyone whether or not they were Waitrose customers."

At that public inquiry, Tucker Parry Knowles, agents for the Queen's grocers, said: "It is proposed that the parking spaces within the Waitrose car park will be free of charge for the first two hours of any stay and will therefore provide ample free short-stay parking for all members of the public irrespective of whether they are shopping at Waitrose."

After giving that guarantee, the agents' evidence went on to say: "The proposed order is both necessary and desirable to enable wider improvements associated with the proposed Waitrose development to be realised.

"The inspector is therefore requested to recommend to the Secretary, or State, that the order should be made."

Mr Clarke, chairman of Monmouth Archaeological Society, said: "This promise in favour of Waitrose must have influenced the decision of the inquiry to allow Waitrose to acquire one of Monmouth's medieval open fields.

"And now they are ripping up their promises and treating them as if they never existed.

"This is not the behaviour we might expect from a company boasting royal warrants."

Mr Clarke has on file all the documents from the inquiry. He says they also show that Monmouth Borough Council repeated those free-parking pledges in its support of Waitrose.

He said: "Their successor, our Monmouthshire County Council, must stand by those promises and enforce them.

"It appears to me that Waitrose was willing to promise anything to get their way on one of Monmouth's medieval open fields – and hope that one day 20 years later we will have forgotten their promise."