Voluntary guidance aimed at making the Welsh Parliament more representative of the population lacks any real teeth, Senedd members warned.
Sioned Williams described new draft guidance for political parties on diversity and inclusion as "too little, too late", with the next Senedd election less than 18 months away.
Plaid Cymru's shadow social justice secretary said the voluntary guidance has no teeth after the Welsh Government scrapped a proposed law on gender quotas in September.
She said: "Although any and all steps to try to increase diversity in politics are very welcome, I am deeply disappointed at the rolling back by the Welsh Government."
Ms Williams suggested a once-in-a-generation opportunity to ensure a 50:50 gender split may have been missed, given the current progressive majority in the Senedd.
'Limited'
She said: "Candidates are already declaring their intention to stand."
"Voluntary guidance, which won't be ready for months, really isn't going to achieve that change for the Senedd elections in 2026."
Her Plaid Cymru colleague Heledd Fychan pointed out there will be no penalties nor repercussions if political parties do not implement the guidance.
The Conservatives' Darren Millar criticised a lack of engagement from the Welsh Government since legislation on gender quotas was withdrawn.
He questioned a "limited" focus on other aspects of equality such as age, faith and disability.
Mr Millar, who represents Clwyd West, said: "We very much hope when the final guidance is published it will be reflective of that broader range of characteristics."
'Sizeable risk'
Jane Dodds, the Liberal Democrats' leader in Wales, raised the Electoral Reform Society's warning of a "sizeable risk" the next Senedd could be the least gender-balanced in history.
She told the debating chamber or Siambr: "The evidence is clear: legislative gender quotas are the only way to achieve gender-balanced Senedds."
Ms Dodds, who represents Mid and West Wales, suggested political expediency and party interests were prioritised over women's representation and democratic reform.
Hannah Blythyn, a Labour backbencher who represents Delyn, said making the Senedd more representative not only changes the face of politics but its focus too.
Ms Blythyn said: "We've talked about other protected characteristics and as an out lesbian, I'm very much in the minority in this place still and I'd like to see that diversity grow."
'Huge disappointment'
In a statement to the Senedd on Tuesday, Vale of Glamorgan MS Jane Hutt recognised "huge disappointment" that gender quotas will not be underpinned by law.
Describing the draft guidance as a catalyst for change, the social justice secretary stressed that the Welsh Government remains committed to a gender-balanced Senedd.
Ms Hutt explained the final guidance, which is also aimed at council elections, will be published in spring following confirmation of new boundaries for the 2026 Senedd poll.
Pointing to measures under the Elections and Elected Bodies Act passed in July, she said: "We all know the guidance alone is not enough to ensure we achieve diversity in politics."
She added: "Women are an under-represented majority in this Senedd. By including the section on voluntary gender quotas in the guidance, I want all parties to recognise the importance of considering what action they may take to increase women's representation."
A consultation on the draft guidance closes on January 7th.
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