Changes in the county's demographics have been blamed for chronic shortages in people willing to work as part of retained crews, who allocate their spare time to respond to emergencies when paged.

In an anonymous email to the Chronicle & Echo, a serving firefighter said other members of Northamptonshire Fire and Rescue Service were being put at risk by the shortage, as well as the general public.

He said: "There are regularly at least 10 crews off the run due to a shortage of retained staff. Part of the problem is people living in rural areas, where the retained crews are based, are not interested in joining.

"They are often not the right age group, or they are company directors who don't feel they need this kind of extra work.

"When there is an incident out in the sticks, it should be the retained crews who are first on the scene but often they are not available, so crews from the middle of Northampton have to go instead.

"Time is essential and seconds can make the difference between life and death. Lives are being put at stake here."

Gary Mitchell, Northamptonshire's branch secretary of the Fire Brigades Union, said the situation had been difficult for several years.

"The retained crews are essential and do a very good job," he said.

"But there is a problem with recruiting people who are willing to do it. The fire service needs some investment to solve the issue, and if there was more funding given to maintain whole-time firefighters, it would take the pressure off the retained crews."

Dave Harding, the fire service's head of performance improvement, said the issue of retained availability was a national problem and not one confined to Northamptonshire.

He said: "Due to the nature of the retained firefighting service, it has always been the case that, on occasions, some retained stations or sections are unable to provide sufficient numbers of people to keep their appliance available for limited periods of the day or night.

"There are a whole range of factors that impact on retained availability. Crews can be unavailable due to planned training, sickness absence, family or occupational demands and of course staff turnover.

"The numbers of stations and the periods of time each are unavailable varies on an hourly basis.

"Members of the public need not be alarmed at this however, as whether through operational demands or through non availability of some appliances, we always ensure that firefighting resources are adequate and suitably positioned to maintain our standards of operational response, which are the same for both urban and rural communities."