A national shortage of armed police is leaving Britain vulnerable to terror attack, the Police Federation of England and Wales has said.
Officers are not volunteering to carry guns because they fear being "hung out to dry" and treated like a suspect if they discharge their weapon, the federation claimed.
Its chairman Steve White warned that Government's plans announced last month to train another 1,500 firearms officers to deal with the terror threat looked doubtful.
The warning comes as a survey of 16,800 officers shows that while 40% fear they will be attacked at work, just half this number (20%) have or want personal firearms.
Mr White told the Press Association: "Before we even start talking about recruiting the extra 1,500, we are struggling to fill the vacancies we have currently got because of the lack of understanding and protection that officers would have if they have to discharge their firearm.
"That's what this survey says - we do fear violence, but officers don't want to carry firearms because they are concerned that if they discharge it, they are going to get arrested for murder.
"But it is all connected with how officers are feeling - they are feeling vulnerable, they are feeling under threat. And if they are feeling like that ordinarily, why would they take the extra step to volunteer to carry guns? It is just more pressure, it is just more potential criticism.
"The Government has got to provide a message to the service, not that it has a get out of jail free card, but that officers will be supported appropriately and fairly in order to encourage officers to step forward and take on additional responsibility."
David Cameron announced last month that money will be ring-fenced to boost the number of firearms officers after terror attacks in Brussels and Paris.
Mr White warned there are "worrying" inconsistencies in the service nationally that have left some forces without firearms officers and reliant on neighbouring forces for coverage.
He added: "This isn't necessarily about the individual police officers, this is about our ability as a service to provide the level of protection that members of the public quite rightly expect.
"I think many members of the public think we have a lot more firearms officers than we actually have. I think they probably think we are all equipped with Tasers. It simply isn't the case.
"The inconsistency around the service is worrying, because no longer can we assume that if there is some kind of terrorist outrage that it is going to happen in the capital."
Mr White's comments come days after the threat from Northern Ireland-related terrorism in Britain was raised from moderate to substantial.
He was speaking ahead of the federation's annual conference this week, with Home Secretary Theresa May expected to get a tense reception when she addresses officers on Tuesday.
The federation said officers are being "let down time and again" by ministers who are not providing them with kit such as Tasers and body-worn video.
The poll found that for many police officers, abuse, assaults and the threat of violence have become quite common.
About 44% of officers said they had received verbal threats at least once a month, while 7% said it happened daily.
More than a third (35%) experienced unarmed physical attacks at least once a month, while 6% said they were attacked with a potentially lethal weapon such as a bottle or gun at least once a month.
Police minister Mike Penning said: "It is vitally important that the police and intelligence agencies have the resources and kit they need to respond to the evolving threats we face. Following the terrorist attacks in France last year, we have announced an additional £143 million over the next five years to provide a national uplift in armed policing capability.
"We ask our firearms officers to do a vital and uniquely challenging job on behalf of the public -- to put themselves in harm's way, in situations where they may have to decide in a split second whether to shoot and, in certain circumstances, to take the life of a person who poses a threat to others.
"We will always ensure firearms officers are supported to take the difficult decisions necessary to protect the public, and that is why, earlier this year, the Home Secretary commissioned a review of the legal and procedural framework governing police use of firearms and post-incident investigation, which is ongoing.
"However, there cannot be any question of police officers being exempt from the normal requirement of the law that any use of force must be proportionate, justified and reasonable."