Counter-terrorism teams could be waiting for up to two years before the extra firearms officers promised to combat an attack are operational, a police group has warned.
Chairman of the Police Federation of England and Wales Steve White said the "best case scenario" would be 24 months to get the additional 1,500 marksmen in place.
Some forces are also struggling to get the right quality of volunteers to go through the rigorous selection and training process.
White said: "Some forces are getting volunteers coming forward, but they are not always being selected because they don't meet the criteria. It is vitally important that standards are maintained. The best case scenario is two years in terms of recruiting an extra 1,500 officers.
"If there is an attack it is unlikely to be an isolated incident. We've got to have the resources around the country because it might happen in multiple places at the same time."
Home Office figures for the year to March 2016 showed that the number of armed officers dropped by eight, but police chiefs have insisted that forces are on track to get the extra marksmen in place over the next 18 months.
Plans to boost the number of armed police were put in place in the wake of the terrorist attack on Paris at the end of last year, when 130 people were killed.
Experts have said that police in France, which has been hit by a spate of terrorist atrocities, have struggled to contain the threat, partly because of their lack of community relationships.
On Monday, Metropolitan Police assistant commissioner Mark Rowley praised members of the public for contributing to efforts to counter extremism and terrorism.
This included an average of 32 calls a day to the confidential anti-terrorist hotline to pass on information about suspicious activity as well as referrals about potential radicalisation or extremist material on the internet.
The authorities are drawing around 60 or 70 people away from extremism every month, Rowley said, and around one in six of those referrals come from the public.