Surgery is being farmed out to the private sector because of a plumbing problem which led to raw sewage leaking down hospital walls.

The head of NHS Greater Glasgow and Clyde, Robert Calderwood, said 18 leaks or floods in as many months had led to a backlog of operations at the ageing Institute of Neurosciences.

The Evening Times revealed in March how surgeons were refusing to operate because of the infection risk.

Five theatres were closed and all elective surgery was cancelled at the unit, which is based at the Queen Elizabeth University Hospital but is not part of the new building.

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While repair work has now been completed on the unit's plumbing network, Robert Calderwood, Chief Executive of NHSGGC, said "additional internal capacity and the private sector" was being utilised to help drive down the backlog of operations.

Mr Calderwood said: “It’s a 1970s building with four inch diameter pipes. We have connected modern equipment to old pipes.

"We had 18 floods or leaks in 18 months. Neurosurgeons were of the view it put patients at risk of infection.

"Dirty water and raw sewage is far from satisfactory. Surgeons demanded this and refused to work.

"We have seven theatres in use now but we have a backlog now of elective care."

Mr Calderwood said signs had been placed around the unit to remind staff not to flush inappropriate items down toilets, to help easy pressure on the ageing pipework.

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He said: "U bends were blocked, above the theatres. Pressure built up and we had leaks.

"There are signs to remind staff what should and shouldn’t go down."

Jennifer Armstrong, Medical Director of NHSGGC said: “Surgeons have been flexible and working extremely hard to get over this.”

A spokeswoman for NHSGGC said: "Extensive remedial work was undertaken to address the problem and bring the theatres back into operation. That work was completed at the end of May at the theatres are now fully operational.

"During the period the five theatres were closed we utilised theatres in other units including the Queen Elizabeth University Hospital complex and the existing two theatres in the Neurosciences building continued to be used for emergency procedures which were being carried out as normal.

"However, this did result in a number of elective procedures being postponed.

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"We now have the theatres back in use and are working to provide appointments as quickly as possible for those patients whose procedures were postponed through additional internal capacity and, in some instances, use of the private sector where appropriate."

The unit was not upgraded when the £842 million new hospital was built and its operating theatres are due to be replaced next year.

The Board has awarded a £7 million contract to build a new state of the art theatre suite which will be ready in early 2017.

The existing theatres will be refurbished for alternative uses.