Lib Dem MP Alan Beith has asked the Competition Commission to investigate The Co-operative Group, claiming it is establishing a local monopoly by buying up rival stores with the sole intent of closing them.

Beith, MP for Berwick, said he wrote to the Commission after receiving complaints from constituents in Northumberland about the Co-op's purchase of a Spar store in the village of Rothbury.

The outlet is to close next month, with staff moved to a nearby Co-op store.

The deal is the latest by the society to provoke anger in Northumberland. Last year, it bought a 1,450 sq ft Londis store in Seahouses, closed it down and moved staff to its shop on the same street. It also recently purchased a 1,500 sq ft Spar in Belford. This was also closed and staff moved to a Co-op store close by.

"The Co-op is acquiring a monopoly in some areas of Northumberland," said Beith. "There seems to be an aggressive policy of driving out competition. If it is true, it flies in the face of the Co-op's fair trade policy.

"I have written to Co-op management and I wasn't happy with the response I got. We had to chase them up for a response and then, basically, they just said they'd done nothing wrong.

"I've also written to the Competition Commission as I think this is an issue for them to investigate."

A Co-op spokesman said: "It is our understanding that the Spar was not trading well and its future was uncertain. The owners of the Spar approached The Co-operative Group requesting that we buy the business. We were able to do so and are delighted to have secured the employment of the staff.

"The owner of the building where the Spar has been trading will seek a new tenant but a restriction will be in place that it cannot be used as a supermarket, convenience store or off-licence. This is normal industry practice.

"While the Spar store will close, the Co-op is not in a position to prevent other food retailers from opening in Rothbury, or anywhere else."

The Competition Commission confirmed it had received Beith's letter and said it would consider the matter in its wider ongoing inquiry into the whole groceries market.