Devon and Somerset's fire services have moved closer to merging next year.
The government says a centralised operation is more efficient
Both chief fire officers have backed proposals which they say will save £3m over five years and leave more money for community safety work.
Devon's Paul Young and his Somerset counterpart Clive Kemp say it will not affect frontline services or result in compulsory redundancies.
But union officials said the statement, made without their agreement, made a "mockery" of the consultation process.
The move follows pressure from the government to make forces in England and Wales more efficient.
The new amalgamated service would have almost 2,000 uniformed and 220 non-uniformed staff.
Devon Fire and Rescue Authority will decide whether to back the move on 25 April and it will be considered in Somerset the next day.
Bernard Hughes, chairman of the Devon Fire and Rescue Authority, said: "This historic opportunity needs to be grasped now in order to both improve services for the future and protect the interests of the council tax payer."
Jim Mochnacz, portfolio holder for Somerset County Council, said: "The growing demands on the fire and rescue service requires that we consider different ways of delivering our services.
"Combination is right for the service as well as for the communities that we serve."
Trevor French of the Devon Fire Brigades Union, which has been involved in consultation on the move, declined to make a statement until he had spoken to Mr Young.
He said: "We are surprised that they have put out this statement.
"It makes a mockery of the whole consultation process."
He said the union had seen no proof of future employee numbers and had received no confirmation that frontline services would not be affected.
He said: "We cannot support the business case because it is not evidence based.
"We find it hard to believe that they will not be cutting frontline services."