Image copyright Reuters Police officers in rural areas fear they would be "sitting ducks" in the event of a terrorist gun attack in the UK, a Police Federation chairman says.
A national shortage of armed officers could leave police in isolated areas "unarmed and vulnerable", John Apter, head of the Hampshire branch warned.
Figures revealed the number of firearms officers in England and Wales last year fell to its lowest level since 1987.
Home Secretary Theresa May will address the Police Federation conference later.
The federation, which represents constables, sergeants and inspectors in England and Wales, is holding its annual conference in Bournemouth.
Do police have firepower to tackle gun menace?
Speaking to BBC Radio 4's File on 4, Mr Apter said armed officers could be more than 70 miles away if an attack was launched in a rural location.
Some rural and coastal areas include potential terrorist targets - including energy and power plants.
"Being realistic, if a firearms unit was coming from the middle of the county you are still talking about 30 miles away - you are not talking about a few minutes," he said.
"There would be an understandable delay.
"If a firearms unit is the other side of the county they could be 70 miles away so you are talking a significant distance.
Image copyright Thinkstock"So the only officers that you have available are unarmed and vulnerable officers and they are the officers that are saying to me that in a terrorist situation they would be sitting ducks."
Last month, Prime Minister David Cameron announced money would be ring-fenced to boost the number of firearms officers in the UK, following terror attacks in Brussels and Paris.
However, federation chairman, Steve White, said the government's plan to train another 1,500 firearms officers to deal with the terror threat looked doubtful.
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, he said.
The result was "worrying" inconsistencies in the service nationally, leaving some forces without firearms officers and reliant on neighbouring forces for coverage, Mr White added.
A national shortage of armed police was leaving the UK vulnerable to terror attacks, he warned.
No 'magic pot'
Che Donald, from the Police Federation, told the BBC the recruitment drive will create a "gap" in the front line, as armed officers transfer from neighbourhood policing.
"There isn't a magic pot we can just dip into and pull out a fully trained firearms officer," he said.
Police minister Mike Penning said the government had committed an additional £143m over the next five years to provide "a national uplift in armed policing capability".
Downing Street said police forces were recruiting more than 1,000 armed officers, setting up round-the-clock specialist teams outside London.
More than 40 police armed response vehicles will also be deployed.
"We will always ensure firearms officers are supported to take the difficult decisions necessary to protect the public," he said.