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 have 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 weaknesses in arrangements for assessing an individual's medical suitability to have a firearms licence, saying they are less rigorous than the checks conducted on prospective bus drivers.
Investigators also found gun owners were illegally allowed to keep using their weapons because of application backlogs and 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 forces are "sometimes inexcusably compromising public safety" by failing to follow government guidance.
He said:"Firearms licensing is not an area which police forces can afford to get wrong. 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."
HMIC highlighted "fundamental gaps" in the system for checking the health of an applicant.
Currently police can obtain permission from the applicant to contact GPs 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 been granted.
The report called for new rules, pointing out that licences to drive a public service vehicle such as a bus require applicants to undergo medical assessments by law.
Authors highlighted the case of Harold Ambrose, a dementia sufferer who killed his wife Wendy with a licensed shotgun in May 2014, before taking his own life. Neither his GP nor local health trust informed Essex Police that his mental health was deteriorating, and they were under no obligation to do so.
Seven out 11 forces inspected experienced backlogs with renewal applications. In one example, two certificate holders were told they could carry on using their firearms legally in their force area even though their licences had expired.
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 probe also found inconsistencies in how forces decide whether a certificate should be refused or revoked.
In one case a man used his certified shotgun to shoot leaves from trees on his property, which bordered the field of a school which was empty at the time. After concerned neighbours contacted police they concluded that his actions did not meet the grounds for revocation.
The report also said:
:: 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, adidng: "What is highly likely is that, if change is not effected, there will be another tragedy."
Chief Constable Andy Marsh, National Police Chiefs Council lead on firearms licensing, conceded there has been a "lack of consistency" among some forces.
He added:"This is something we will play our part in addressing, but it must also be noted that, with over 30 pieces of legislation governing firearms licensing, this means that navigating these laws leaves too much room for ambiguity and inconsistency."
Policing minister Mike Penning said firearms licensing is kept under review and talks have started to ensure appropriate information sharing between GPs and police.
"The UK has some of the toughest gun laws in the world and we are determined to keep it that way," he said.
Last year there were 734,000 holders of firearms certificates, covering 1.8 million shotguns and other guns.