The warning comes after the announcement on Saturday that firefighters in England and Wales will strike on 19 October over what the union has described as a "ludicrous attack" on firefighters' pensions.
The strike by union members will last for five hours, between 18:30 and 23:30.
The Government plans to make firefighters work until they are 60 and argues that the pension offer is generous.
But FBU general secretary Matt Wrack argued late-50s is too old to be fighting fires and rescuing people.
"We had hoped our first strike was enough to show Government that firefighters could not be more serious about protecting public safety and ensuring fair pensions," he said.
"No firefighter wants to strike, and it's desperately disappointing that Governments in Westminster and Cardiff continue to deny reality over pensions costs and the need for a pension scheme that reflects the job firefighters do."
Wrack added: "We hope this second strike will mean both Governments will be willing to discuss the full range of concerns that firefighters and the general public have expressed.
"However, more strikes cannot be ruled out if that's what it takes to protect public and firefighter safety from these ludicrous attacks."
Firefighters in Scotland will not strike after they voted in a consultation that Scottish Government proposals were sufficient to prevent a strike, the FBU said.
During the first strike on 25 September, 32,000 union members walked out for four hours, according to the FBU.
