A shortage of frontline staff and lack of training could be putting the public at risk the Fire Brigades Union (FBU) has said.
The FBU says firefighters whose training is not always up to date or adequate are being sent to fight fires which is "endangering the public" in the Mid and West Wales Fire and Rescue Service region.
The fire service denied using untrained staff on fire engines but admitted "crewing challenges".
The union said that up to 21 of the region's fire engines - around 30% of the fleet - are unavailable daily because, it claims, there are not enough trained fire fighters to run them.
Barrie Davies, Mid and West Wales FBU brigade secretary, said: "Techniques in firefighting change fast, and knowledge decays. If an operational firefighter does not go out and fight fires for six months, he or she is rightly expected to do a two week rehabilitation course before being sent out again."
Mr Davies said that the fire service in west Wales proposes to send people out on fire engines who have not had to do so for up to 20 years.
"However willing and able such people are, they are simply not safe, through no fault of their own.
"This situation is being exacerbated by the fact that part-time firefighters are being taken from their own communities to cover full-time deficiencies, a case of robbing Peter to pay Paul.
"I think the Fire and Rescue Service panicked when it realised how bad the shortage of frontline firefighters actually was, and it is now putting both its firefighters and the public in danger by taking this course of action."
It raised concerns earlier this year when the service bought 17 vans to replace some traditional fire engines.
Deputy chief fire officer Paul Bates said: “We recognise that we currently have some crewing challenges at certain times of the day, in some areas of the service. But to suggest that up to 30% of our operational fleet is unavailable is not true and misleading."
He said that staff were being utilised more flexibly and highly trained and skilled fire officers are performing their normal roles but at stations where there are crewing difficulties, enabling appliances to be kept available and ensure strategic fire cover across the whole area.
“All personnel are competent to perform the skills required of them in the roles they are currently performing, both in terms of their roles in community and business fire safety departments and as officers in charge of crews at incidents.
“This flexible deployment of staff across the Service area is an everyday occurrence and ensures we have the right people in the right place at the right time," Mr Bates added.