Researchers at the University of Dundee found that routine mortality monitoring in general practice was only likely to raise alarm about another murderer like Shipman once they had already completed "dozens" of killings.

The researchers, led by Professor Bruce Guthrie at the Tayside Centre for General Practice, examined the effectiveness of routine mortality monitoring - one of the measures recommended by the Shipman Inquiry to increase the chance of deterring and detecting a future serial killer.

The study found that even an extreme killer like Shipman may only be picked up by the system when they had already killed repeatedly.

The report, published in the British Journal of General Practice, gave a figure of more than 30.

It said: "A monitoring system that can only detect a serial killer after 30 or more people have been murdered is not 'effective' in any meaningful way, and represents a failure of other mechanisms intended to detect and deter murderers by ensuring that every death is properly accounted for."

Professor Guthrie added: "It is extremely difficult to establish an effective monitoring system like this and there are clear doubts over how effective it can be.

"Based on our analysis, monitoring mortality rates is not enough and cannot substitute for other recommended reforms, such as an effective reform of the coroner and death certification systems which were also recommended by the Shipman Inquiry but have not been pursued."

The researchers used statistical data from the NHS in Scotland to examine coverage provided by a mortality monitoring system and how successful it would be in detecting murderers.

They found that monitoring practices even over a three-year period was severely limited by the relative instability of medical working patterns - in Scotland 40% of practices have at least one GP arrive or leave in that timespan.