Inexperienced police officers, like the two involved in the Bradford shooting, are often out on patrol unsupervised, according to a leading police figure.
Pc Beshenivsky had been a serving police officer for nine months
Alan Gordon, vice chairman of the Police Federation, said a major recruitment drive in recent years had left forces across England and Wales with a large number of new, and
therefore inexperienced, constables.
Mr Gordon said it was a phase any organisation could go through which would eventually "plateau out".
Pc Sharon Beshenivsky, 38, was shot dead and her colleague, Pc Teresa Milburn, 37, was injured during a robbery at a travel agents in Bradford on Friday.
'Lives on the line'
Pc Beshenivsky, who died after being shot in the chest, had been a serving police officer for nine months, while Pc Milburn, who was shot in the shoulder, joined West Yorkshire Police in April 2004.
In the wake of the shooting Home Office minister Hazel Blears said what happened in Bradford highlighted "the fact that police officers right across the country put their lives on the line in many cases in order to protect the rest of us".
The number of police officers in England and Wales has risen by more than 10% since Labour came to power in 1997.
Home Office figures show in 2001 there were fewer than 130,000 officers in England and Wales, while there are now more than 141,000.
Mr Gordon said it was not unusual for relatively inexperienced officers to be out on patrol unsupervised.
More than 10,000 extra officers have been recruited since 2000
He said: "With our recruitment drive we've got more officers than ever and as a consequence we have an unusual number of probationary constables.
"We have insufficient numbers of experienced officers to accompany them all the time."
New recruits go through a two-year probationary period during which they go through training at a residential course and on the job, working alongside more experienced officers.
They will typically spend 10 weeks on patrol with a tutor constable and then, after 32 weeks of training, can be allowed to go out on patrol alone - if deemed suitable by their divisional commander.
Mr Gordon said in addition to there being many new recruits, a lot of experienced staff had been taken away from front line activity to work on important projects away from "24-7 policing", leaving less-experienced officers out on the streets.
He said: "It's an unusual phase we have to go through but it will plateau out eventually, as a rising number of police officers become experienced.
"It's a phase any organisation would go through if it recruits heavily."
West Yorkshire Police said its investigations into the shooting included looking at whether the seriousness of the incident was known when the constables went to the scene.
Chief Constable Colin Cramphorn said he had "searched his soul" about what could have been done to prevent Pc Beshenivsky's death and Pc Milburn's injuries, but concluded the officers had the right protective equipment and back-up to deal with the robbery.