An investigation into what caused thousands of elective operations to be cancelled at Nottingham University Hospitals Trust has blamed the hospitals’ own reorganisation of services.

An external report commissioned by the trust concluded the “most important single contributor” to the crisis earlier this year was the switch of emergency services and elective activity between Nottingham’s City Hospital and the Queen’s Medical Centre during 2011.

Between January and August this year the trust cancelled 870 operations on the day of surgery, and 2,891 operations before the day of surgery.

Earlier this month HSJ asked trust chief executive Peter Homa what had caused the problem. He replied: “We’ve seen an increase in the acuity of emergency patients at the QMC with longer lengths of stay.”

But the report, by consultancy Mott MacDonald, concluded these factors “played a relatively minor part in the system deterioration”.

Instead it found the reorganisation of beds led to a “loss of resilience” in the system and removed what it called a “buffer” bed capacity. The system was then unable to cope with peaks in emergency admissions and elective work.

The report said: “This caused the system to behave in an unexpected manner, rendering forecasts and planning assumptions invalid. This was a novel situation that could not have been predicted from previous experience.”

But it also found reporting systems at the trust were not sensitive enough and “did not pick up early warnings”.

Although the trust monitored its on-the-day cancellations it had been unaware of the large number of operations cancelled prior to the day of surgery for several weeks.

The report found the reorganisation led to a loss of bed flexibility at the QMC and forced the trust to cancel operations which it then rescheduled as soon as possible, creating what the report called a “swing in the pattern of flow into and out of hospital beds”.

“Such a system is unstable, difficult to control, and lacks resilience if faced with short-term increases in demand,” it said.

In a statement the trust said: “We are planning for this next winter, taking into account the lessons of 2011-12 and the report’s recommendations.

“We have changed our approach to planning our bed requirements for emergency and elective patients, have better reports of cancellations and flow in and out of our hospitals, and we are on track to roll-out a real-time electronic bed management system across our wards.”