The government's plan to replace England's 46 fire control rooms with nine regional centres will cost lives, the Fire Brigades Union has warned.
Ministers say the control centres will be more efficient
The FBU said the changes could also hamper the emergency response to any future terrorist attack and that frontline jobs will be cut.
Ministers said earlier in August that the £1bn regional centres plan would go ahead, despite opposition.
They said the changes would make fire service responses more efficient.
Seven of the new sites were announced on 10 August. They are in Durham, Warrington, Cambridge, Leicestershire, Wolverhampton, Wakefield and Taunton.
A site in the South East is still to be confirmed, while one has already been set up in London.
But FBU president Ruth Winters, speaking at a meeting of union members in Bournemouth, rejected the claim that response times would be improved.
"It will make the response less efficient and it will cost lives," she said.
A regional centre is already operating in London
"The government wants to introduce huge multi-brigade controls, with fewer people and roomfuls of glossy, expensive, but untried technology.
"To depend on technology instead of depending on people is folly."
The closure of the existing control centres will cost up to 380 jobs, but will save around £20m a year, ministers have said.
Chief fire officers have broadly welcomed the plan, but the union fears the scheme will be paid for by cuts to frontline jobs.
Fire Service minister Jim Fitzpatrick has said it is hoped compulsory redundancies would not be needed.
He said the new centres would help frontline officers by giving them access to information about an incident before they reached the scene.
He said the recent London bombings had shown the need for control centres able to deal with terror attacks or natural disasters.
The planned site of the South East regional centre is not yet known
"While existing control rooms do a good job, they are not designed to deal in a co-ordinated way with major regional or national incidents," he said.
A spokeswoman for the Office of the Deputy Prime Minster denied frontline jobs would be cut.
"It is regrettable that the FBU are continuing to scaremonger," she said.
"Their totally unfounded allegations betray a complete lack of understanding of the government's plans for control centres."