Lieutenant Colonel Tim Hodgetts, a special adviser in accident and emergency medicine, will tell a conference that the people who usually provide emergency cover at big events may prefer to join the celebrations themselves for this New Year's Eve.

St John Ambulance volunteers and off-duty doctors, nurses and paramedics might not want to give up their free time on such a special occasion or simply may not be available, he will say.

But a spokesman for St John Ambulance said the organisation does not anticipate any problems at the moment.

It has, however, been preparing for New Year's Eve for some time, he said.

Hospitals fear staff shortages

Hospitals have already expressed concern that they may not be able to cope with a major incident over the millennium because of their low staff levels.

Dr Ken Hinds discusses the importance of planning for the millennium

The NHS Executive has advised them to put all applications for leave on hold until they know what their staffing requirements are.

But Lieutenant Colonel Hodgetts fears the impact of this on care in the field.

He organised Thursday's Pre-Hospital Immediate Care Conference in London to examine the issue.

Doctors, emergency services and event planners are expected to attend.

Events rely on volunteers

The Lieutenant Colonel said: "The problem is that major events, like the celebrations in Scotland and concerts and gatherings at places like Trafalgar Square, depend largely on volunteers.

Trafalgar Square in London is likely to be mobbed on 31 December

"They may range from the plumber who volunteers for St John Ambulance in his spare time to off-duty doctors, nurses and paramedics who give their services free and sacrifice their own night out.

"The fact is that it is a very real problem that these people might not want to do that on this New Year's Eve.

"There is only a finite number of people who have the first aid training and if there are not enough of them there could be major problems.

"The other problem is that the trained volunteers - for instance doctors and nurses, may already be committed to working because of the increased staffing levels that will be needed in accident and emergency departments."

Recommended levels of medical cover

The first aid requirements for large events are set out in the so-called Purple Guide. It recommends that there should be:

• One first aider per 1,000 people, up to a crowd of 20,000
• One first aider per 2,000 in larger crowds
• One doctor per 2,000 people
• One paramedic ambulance per 5,000 people

It is estimated that an average New Year's Eve celebration in somewhere like Trafalgar Square sees nine casualties per 1,000 people.

Half of these will be injuries and the other half medical problems like asthma attacks, low blood sugar and fainting.