Councillor Iain McIntosh, cabinet member for housing, planning and economic development, had declared an interest, which meant he would not be voting on the proposal and spoke against it.
Powys County Council’s cabinet met on Tuesday to look at the consultation report to reorganise schools in and around Brecon.
Following a unanimous vote in favour of merging Mount Street Infants and Junior schools - as well as Cradoc Community Primary School - under one headteacher and governing body and eventually moving from three sites to a new-build school, Councillor McIntosh dropped his bombshell.
Councillor McIntosh said: “This is a decision I cannot accept and I don’t recognise it as a Conservative party or group decision, so I’m afraid I’m going to be resigning from the Conservative group with immediate effect and obviously my position on the cabinet as well.
“This is a decision I just cannot stomach and will be fighting very much against it from this point.”
Earlier in the meeting, Councillor McIntosh had pointed out that both Conservative elected members for Brecon and Radnorshire - Fay Jones MP and James Evans MS - were against the proposals.
School reorganisation has been championed by Conservative education portfolio holder, Councillor Phyl Davies.
Council leader Councillor Rosemarie Harris stressed she wanted to discuss the issue with Councillor McIntosh.
Councillor Harris said: “As the leader of the cabinet and the council, I would like to speak with you outside of the call.
“You have a tremendous amount to offer this council and I would ask you to reflect on that.
“I understand that feelings are running high today and I would appreciate a discussion.”
The proposals are for a new 360-place English-medium primary at Penlan, the former site of Brecon High School.
A Pupil Referral Unit (PRU) and a community swimming pool would also be part of the scheme which could be built on the site of the old Brecon High School.
Consultation on the proposal took place between February 25 and May 12.
Merging the Mount Street schools and Cradoc into a 'new primary school' would happen by September 2023. Pupils would then transfer to a new school building in 2025/26.
Earlier in the meeting, the council’s head of communication and transformation, Emma Palmer, had told councillors that eventually “years down the line” the expectation is that the new-build joins with Brecon High School to become an “all through school”.
The whole scheme, which includes a new 180-place dual-stream primary school in Sennybridge - replacing the current building there - is estimated to cost £32m.