Reform UK Wales member Joe Martin, party leader Helen Jenner and chief whip Llŷr Powell announced the boycott on the Senedd's steps
Reform politicians will partially boycott the Senedd to protest the ban of one of their politicians for claiming white people are being "replaced" in the UK.
The party's Welsh leader Helen Jenner claimed Joe Martin's ban was because of "undue pressure" from governing party Plaid Cymru and was an "infringement on freedom of speech".
Martin had been told he would not be allowed to speak in the Senedd's chamber until he apologised for the remarks, which the parliament's Presiding Officer Huw Irranca-Davies called "unnecessarily provocative and deliberately offensive".
The party said its members would withdraw from most of Wednesday's proceedings before returning to vote later in the evening.
Earlier on Wednesday Jenner said in a statement that freedom of speech was "a fundamental part of our democracy".
"It means very little if it only applies to opinions we agree with," she said.
"We cannot allow the Senedd to become a place where challenging views are shut down."
Martin was also backed by the leader of right-wing party Restore Britain, Rupert Lowe.
On X, Lowe said the Welsh Parliament should be "shut down" if it was "going to shut down legitimate debate around demographic security and immigration".
Martin's Caerdydd Penarth constituency includes some of Wales' most ethnically diverse communities.
Joe Martin was told he would not be called to speak until he apologised
He asked for a Welsh government statement on the "demographic replacement of white British people" during business questions, when Senedd members are free to raise issues they choose.
Business Minister Heledd Fychan immediately asked the presiding officer whether the remarks were within the Senedd's standing orders - a set of rules that govern conduct in the chamber.
Standing orders give the presiding officer discretion to decide if contributions to plenary sessions - meetings of the full Senedd - use "discriminatory or offensive language".
Delivering his ruling later, Irranca-Davies said: "Until I receive a clear indication that the member understands the boundaries of acceptable parliamentary conduct and apologises to the Siambr [chamber], we will not be calling him to speak."
Senedd officials confirmed an apology would have to be made in the chamber during a plenary session.
The ruling does not cover committees.
Martin is a member of the Senedd's Equality, Human Rights and Social Justice Committee.
Irranca-Davies pointed out it was not the first time he has rebuked Martin. In September, he said Martin had made offensive remarks about surgeons working in the Welsh NHS gender service.
Plaid Cymru MS Marc Jones said Martin should apologise "for the constant harm and fear he causes among our communities".
In the 2021 census, 93.8% of people in Wales identified as white, down from 95.6% a decade earlier.
The proportion identifying as Asian, Asian Welsh or Asian British, the next highest group in the survey, rose from 2.3% to 2.9%.
