Didcot's £50m town centre redevelopment could be knocked off course by a High Court battle.

The bombshell has been dropped by the Co-op, which is challenging a planning decision by South Oxfordshire District Council to allow Sainsbury's to add a further 20,000 sq ft of floor space to its 50,000 sq ft store at the new Orchard Centre.

The Oxford, Swindon & Gloucester Co-op, which has a supermarket in Market Place, on the edge of the Orchard Centre site, has lodged an application in the administrative division of the High Court for a judicial review of the planning decision. If successful, it could prevent the first-floor extension to the Sainsbury's store.

Developer Taylor Woodrow, which submitted the planning application for the enlarged store, could make a new application. The store could go ahead with its original plans for a one-storey store without delay.

Capt John Flood, district council cabinet member for Didcot town centre, described the legal action as "bloodymindedness on the part of the Co-op".

Co-op spokesman Adrian Barradell said: "The reason behind our application for a judicial review is simply down to the sheer size of the planned store."