Four councillors will form a Labour group within Powys Council on Monday, ending decades of non-partisan local politics in the county.
Powys is believed to be the last non-political council in the UK
It is thought to be the last local authority in the UK without political groups, and the move is expected to prompt other parties to form.
The Labour group claimed the group system would be more accountable.
But other councillors said party politics would interfere with the smooth running of the council.
Since 1974, councillors' independence has meant they have not had to toe a party line when voting.
Some council members claim this non-confrontational approach has made for transparent debate and clarity, and is the envy of other local authorities.
But the leader of the council's Labour group Sandra Davies disagreed.
Labour's decision could prompt the Liberal Democrats to form their own band of up to 24 members, but the independent councillors will be the majority.
However, it remains to be seen if they, the Conservatives or Plaid Cymru decide to form their own groups. There are 73 councillors in all.
Mrs Davies, who has been a Powys councillor for 18 months, gave her reasons for altering the political landscape in Powys.
She said: "We intend to declare after the meeting.
"We think the council would be more accountable and more efficiently run if it had political groups.
"Powys is an enormous county and I represent a ward (Cwmtwrch) in the Ystradgynlais area (near Swansea).
"People there feel out on a limb and on the edge of the county. All the Labour councillors are from that area and we feel we can be more accountable to our constituents by forming a group."
Liberal Democrat councillor Gareth Morgan, who sits on the council's ruling board, is against politicisation.
He said: "When I was chairman of the council a few years ago and met colleagues from other local authorities from Wales, it was significant that they envied the system in Powys where everyone worked together .
"They liked that we did not let political parties interfere with policy and that meetings were transparent.
"I've been a councillor for 32 years and I find it very sad - it will be the end of an era. I am very reluctant to take any step that will introduce any groups - my heart is not in it."
Conservative Mid and West Wales AM Glyn Davies said it seemed "inevitable" that the council would alter, but at least there would be "genuine opposition" to the "ruling elite", he added.
He said: "I have an emotional attachment to the idea of a wholly 'independent' council where councillors are answerable to the constituents rather than to their political groups."
The Local Government Association said it believed Powys was the last remaining non-political authority in the UK.