Covid passes will be required in nightclubs and large events in Wales from 11 October after Welsh ministers won a vote of the Senedd.
In a knife-edge vote the measures were agreed with 28 politicians voting for and 27 voting against.
The public will now be expected to show evidence of being fully vaccinated or having a recent negative Covid test.
The Conservative and Liberal Democrats were already known to be voting against the plans and Plaid Cymru announced this evening they would be to voting against the Welsh Government.
Health Minister Eluned Morgan told mmbers not supporting the law “will be an act gross of irresponsibility”, but Plaid Cymru had said it was open to exploitation.
Confirming their voting intention on the latest Welsh Government regulations, Plaid Cymru Spokesperson for Health, Rhun ap Iorwerth MS said: “Since the start of this pandemic, Plaid Cymru has been ready to support the introduction of a range of measures to control and limit the transmission of the virus – even in situations where sometimes quite severe limitations have been imposed on our freedoms.
“When we have backed restrictions, it’s been when we have been convinced that the evidence is clear on the positive impact those measures would have, and that they can be implemented in practice.
“Regrettably, the regulations proposed by Welsh Government today raise more questions than they provide answers. There is insufficient evidence and little detail on how it will work in practice. In particular, the rapid test loophole which represents a fudge on the part of the Government, makes the system open to exploitation.
“We’ve asked many questions and haven’t been given the assurances we’ve sought. And it’s for that reason that we feel unable to support these regulations today.
“We are not voting against because of issues of principle. We asked Welsh Government to withdraw this motion and made it clear we’d contribute to discussions on how something more robust could be brought forward, better evidenced, and with clarity on implication. That offer is still open.”
If all opposition MSs had taken part the government would have lost in a tie.
A Conservative source said a member of the group had been unable to log on to take part in the Senedd vote.
Protesters had gathered outside the Senedd’s buildings on Tuesday evening, chanting “shame on you” following the result.