Britain faces new gun massacres unless the chaotic firearms licensing regime is overhauled, a police watchdog has warned.

The public are at risk because of a litany of failures and inconsistencies in the way prospective and existing gun-owners are vetted and monitored, inspectors found.

In a highly critical report, Her Majesty’s Inspectorate of Constabulary (HMIC) said lessons had not been learned in the wake of tragedies such as the killing of 12 people by taxi driver Derrick Bird in Cumbria in 2010.

They highlighted “unsatisfactory” arrangements for assessing an individual’s medical suitability to have a firearms licence, saying they were less rigorous than the checks conducted on prospective bus drivers.

Investigators found gun owners were illegally allowed to keep using their weapons because of application backlogs, while nearly half of all 43 forces in England and Wales fail to contact referees listed by new applicants.

HM inspector of constabulary Stephen Otter, who led the inspection, said: “Firearms licensing is not an area which police forces can afford to get wrong. Public safety relies on it.

“We found that, too often, forces are not following the Home Office guidance that is in place, sometimes inexcusably compromising public safety. Lessons from past tragedies have not always been learnt and this fails the victims of those events, including their families, unacceptably. Unless things change, we run the risk of further tragedies occurring.”

The regime for monitoring applicants’ health was singled out as a major area of concern.

Under the current system, they have to give police permission to contact their GP to obtain details of their medical history. However, doctors are under no legal obligation to respond and police routinely write to them after the certificate has already been granted.

The report said this meant there were “fundamental gaps” in the system and pointed out that licences to drive a public service vehicle such as a bus required applicants to undergo medical assessments by law.

Authors highlighted the case of Harold Ambrose, who killed his wife Wendy with a licensed shotgun in May 2014, before taking his own life. He had been diagnosed with dementia but neither his GP nor local health trust informed Essex police and they were under no obligation to do so.

HMIC called on the Home Office to implement rules so that licences are not handed out without a current medical report from the applicant’s GP and that police are informed of any relevant changes in circumstances.

Seven out 11 forces that underwent detailed inspections were experiencing backlogs with renewal applications. In one example, two certificate holders were told they could carry on using their firearms legally in their force area.

The report said: “For forces to allow certificate holders to possess firearms unlawfully is a serious failure; arguably, they have colluded in breaking the law.”

The investigation found inconsistencies in how forces decide whether a certificate should be refused or revoked. A licence can be blocked if the individual is thought to pose a danger “to the peace” but there is no definition for how this should be applied when assessing revocations or refusals.

In one case a man used his certified shotgun to shoot leaves from trees on his property, which bordered a school field. The school was empty at the time but concerned neighbours contacted police. However, the force concluded that his actions did not meet the grounds for revocation.

The report also found:

• Only four of 11 forces inspected had effective monitoring and audit arrangements in place.

• Just 28 out of 43 forces in England and Wales contact referees for all new firearms certificate applications.

• A total of 656 shotguns or other firearms were stolen or went missing last year.

HMIC issued a total of 18 recommendations to police and the Home Office.

“We cannot make our position any clearer,” the watchdog said. “What is highly likely is that, if change is not effected, there will be another tragedy.”

Last year in England and Wales, there were 734,000 holders of firearms certificates, covering 1.8m shotguns and other weapons. Licences last for five years.