Members of the Fire Brigades Union (FBU) issued the ultimatum yesterday over proposals by North Yorkshire Fire and Rescue Authority to replace six fire engines with smaller vehicles.
The FBU says the smaller tactical response vehicles can carry just two firefighters and the changes could increase response times by more than 30 minutes.
It has threatened to ballot for strike action within seven days if the proposals set out last year are not scrapped.
But the authority said the changes would have "no significant adverse impact" on services and that it had conducted an extensive consultation with the union.
Steve Howley, secretary of the FBU in North Yorkshire, said: “The Combined Fire Authority's (CFA) plan will lead to a significant and unacceptable increased risk to the public and to frontline firefighters.
"Despite public opposition to the plans agreed by the CFA last December and our best attempts to resolve the issues through negotiation with local managers, we now have been forced to put the matter in the hands of the Fire Authority to try to resolve, if they fail to do so, we are left with no option but to ballot our members for industrial action.”
The union has also warned the plans will extend the time firefighters spend performing fire rescues and dealing with traffic collisions, and that crews could have to wait at the scene for specialist equipment from other fire service vehicles to arrive if the changes go ahead.
The authority said it is considering a response to the FBU.
Owen Hayward, North Yorkshire's temporary assistant chief fire officer, said: "The Service believes that all the concerns raised by the FBU have been addressed previously.
"There has been extensive consultation with the FBU over the last two and half years about introduction of the tactical response vehicles but they are unwilling to accept new ways of working.
"Risk assessments and analysis of impact undertaken over the last three years suggest that their introduction will have no significant adverse impact on the service to the public or to the safety of staff."
The union claims the authority promised to phase in the changes over a four-year period, which it says would have given managers time to address concerns.
Mr Howley added: “The CFA have gone back on their word. Instead of trying to fix the problems, senior managers are pushing ahead with a drastic plan which will end up costing lives. We cannot accept that.
"The CFA and chief fire officer for North Yorkshire are failing in their duty to provide the effective fire and rescue service the public pay for and deserve.”
The authority stressed it would make every effort to avoid industrial action or minimise the impact it has on public safety.