Reform UK has formally complained about not being included in a TV debate ahead of the election.
BBC Wales will host a debate on April 29. In the first hour viewers' questions will be put to Labour, Conservative, Plaid Cymru, Liberal Democrats, and Abolish the Assembly.
In the second part of the programme leaders from Reform, Ukip, and the Green Party will be asked to take part.
Reform UK, previously the Brexit Party headed up by Nigel Farage, says it is being treated unfairly and has complained to Ofcom about not being included in the main debate.
In Wales Reform UK is led by former Ukip and Independent MS for North Wales and Brexit Party MEP Nathan Gill. A complaint has been sent to the UK's communications regulator Ofcom.
The letter from the party's legal firm says that the regulator needs to look at the issue "urgently".
"The decision amounts, in our client's view, to a clear breach by the BBC of provisions in sections five and six of the Ofcom Broadcasting Code ('the Code') and the BBC Charter and that unless the BBC is compelled to revisit that decision and exercise its editorial judgement rationally our client will be seriously prejudiced."
It criticises the inclusion of Abolish the Welsh Assembly Party (AWAP) in the programme as that party has 23 candidates in the election while Reform UK has 40.
"The complaint is that it is clearly not appropriate that the leader of the AWAP should be given precedence and able to appear in the programme alongside the other main party leaders while Reform UK is relegated to a lesser programme.
"There is simply no reasonable or rational basis on which the BBC can possibly defend its decision to allow the leader of the AWAP to appear in the programme whilst not allowing the leader of Reform UK in Wales to do the same."
Richard Tice, the party's UK leader, said: "The decision by the BBC to wilfully exclude Reform Wales from the leaders debate on April 29 is obviously wrong.
"Particularly given that Reform – as it was then, The Brexit Party – won a national election in Wales a mere two years ago. Not only that we are running a full slate of candidates in line with the other major parties for the Senedd. Not to include us in this debate is clearly biased, unfair, and totally unacceptable.
"We expect Ofcom to urgently review the BBC decision and for the BBC to stand by its commitment to provide a full and fair opportunity for the people to see fair democracy working in action.
"I hope that this is not some form of latent revenge for that result in 2019."
Mr Gill said: “It is the duty of a public broadcaster to allow the viewers to judge for themselves who they wish to vote for. By excluding Reform UK they are denying the electorate the opportunity to hear from all of the major parties in this election and effectively taking choice away from the people.”
A spokesman for BBC Cymru Wales said: “BBC Cymru Wales will be providing a range of content and debating opportunities on television, radio, and online in both English and Welsh during the forthcoming election campaign.
“As part of our commitment to provide news, analysis, and scrutiny there will be one Welsh leaders’ debate programme on BBC One Wales on April 29 with two debating panels. The first panel will include invited leaders from the Labour, Conservative, Plaid Cymru, Liberal Democrat, and Abolish the Assembly parties. This hour-long debate will be followed immediately afterwards with a second debate which will be around 30 minutes in duration. This panel will feature invited leaders of the Reform, Ukip, and the Green parties in Wales.
“In determining the format and composition of the panels appearing on the programme we were required to make an editorial judgement with regard to what is an appropriate level of coverage. That careful judgement has taken account of past levels of electoral support as well as any trends in current support and political circumstances. We are confident that our editorial processes have been fair and impartial in reflecting the reality of the Welsh political landscape.”