A plan to change Senedd rules on the formation of party groups has been called a "stitch-up" by the Abolish the Welsh Assembly Party.
Labour's proposed rule change, supported by Plaid Cymru, would mean four Senedd members would be required to form a group instead of three.
The Tories and the Independent Alliance for Reform opposed the changes in the Senedd's business committee.
Party groups have access to more staff and greater status in proceedings.
In May 2019, Plaid Cymru, and some Labour politicians, had wanted the rules altered to stop a Brexit Party group from being formed.
Labour, the Conservatives, Plaid Cymru and the Independent Alliance for Reform make up the four groups currently established in the Welsh Parliament.
The Independent Alliance was the last group to be formed, including Members of the Senedd (MS) who had previously been a part of the Brexit Party group, having initially been elected to represent UKIP.
At a meeting of the Senedd's business committee on Monday, the Welsh Labour government's Trefnydd (business manager) Rebecca Evans proposed changing the group threshold to four Senedd members that belong to the same political party that won seats in the previous election.
As part of the proposed changes, the presiding officer could use their discretion "in exceptional circumstances" to allow the formation of groups of four MSs that were not elected to represent the same party.
It is suggested that "exceptional circumstances" would include a split in a major political party, a national crisis or major event that changes group affiliations, or whether a group "has a significant and demonstrable democratic mandate for formation and whether it shares a political philosophy that would be clear to the electorate".
The proposal was supported by Plaid Cymru's Sian Gwenllian but opposed by the Conservatives' Mark Isherwood and Caroline Jones from the Independent Alliance for Reform.
Elin Jones, the Senedd's Presiding Officer, has invited comment on the proposals from the other politicians in Cardiff Bay.
In response, Abolish MS Mark Reckless said it was "inappropriate to give the office of Llywydd such wide discretion, exercise of which would inevitably draw its holder into party political controversy".
"As drawn, the proposed changes to standing orders read as if designed ex-post to prevent the formation of the Brexit Party group, of which I was part," he said.
"I submit that the proposed new standing orders would be a stich-up by Labour and Plaid Cymru to appropriate even more public money for their own groups, while denying it to those they consider a threat or of whom they disapprove," Mr Reckless added.
A Plaid Cymru spokeswoman said: "There are ongoing discussions in the Business Committee and in the groups about potential changes to standing orders (rules)."
'Behaviour of a dictatorship'
A Welsh Conservative spokesman said: "We do not believe this is democratic and made these representations known in business committee.
"The Senedd's standing orders are extremely important in the structure and organisation of plenary business and shouldn't be subject to political gerrymandering in the final days of this parliament."
A spokesman for the Independent Alliance for Reform said: "The changes are politically motivated and not based on political balance.
"The moving of the goal post is a clear establishment stitch up.
"This plan to silence opposition members is the behaviour of a dictatorship and not a nation that believes in freedom and democracy."
Neil McEvoy, leader of the unregistered Propel party, said on Twitter the proposals suggested Labour and Plaid Cymru "are expecting a new, smaller group elected".
"My view is that it may not be so small post-election," he added.
A spokesperson for the Senedd Commission, which is in charge of the day-to-day running of the institution, said: "As part of its review of standing orders in preparation for the sixth Senedd, the Business Committee has recently considered the provisions of standing order 1.3 relating to party groups.
"A proposal to amend the standing order has been made and is supported by a majority of its members. The committee is now consulting with those independent Members not represented on the committee.
"Any comments will be considered by Business Committee at its next meeting. Any proposals to amend the standing orders need to be approved by a resolution of the Senedd, supported by at least two thirds of members."