Civil liberties campaign group Big Brother Watch is seeking a judicial review of the scheme, which it has said is “draconian, discriminatory and pointless”.
The NHS Covid Pass was introduced in Wales on October 11 2021 and allows people to prove they are vaccinated or have had a negative lateral flow test so they can attend large events or venues.
People attending cinemas, concert halls and theatres, as well as nightclubs and certain large events, are required to present a Covid pass.
The Welsh Government said they were introduced to “keep businesses open while also helping to control the spread of the virus and protect the NHS”.
But Big Brother Watch said the reasons behind the scheme “don’t stand up to scrutiny” and wants full disclosure of the documents and advice that led Welsh ministers to implement it.
The group claims the scheme creates “significant and far-reaching” rights interferences.
It has also initiated a legal challenge against the Covid pass scheme in England, and is urging the UK Government to scrap the English scheme by the end of January.
The group’s director Silkie Carlo said: “We support proportionate measures to protect public health, but there is exceptionally weak evidence supporting the role of Covid passes.
“We’re calling on the Welsh Government to disclose the documents and advice they say they’re relying on to impose the Covid pass scheme on people in Wales. Their refusal to do so raises suspicions that the government’s case is weak or non-existent.
“The reluctance to be transparent now even extends to an unwillingness to consent to Big Brother Watch publishing the legal papers from its own claim, presumably because they quote from correspondence that attempts to justify the scheme.”
“If Covid passes are having no noticeable effect on public health in Wales, as appears to be the case, there is simply no justification for them. Mandatory Covid IDs are a disproportionate intrusion of medical privacy, an erosion of freedom and a threat to equality,” they added.
Shirin Marker, the Bindmans solicitor representing the group, said: “It is also important that the legal process on a matter of public interest like this is transparent, which is why we are seeking the court’s permission to allow Big Brother Watch to fully publish details of its claim.”
The Welsh Conservatives are backing the legal challenge by Big Brother Watch.
Russell George MS, Welsh Conservative and Shadow Health Minister, said Covid passes are "coercive, ineffective, costly to business, and continue to have no evidence base."
"We must not forget that they are not a route out of restrictions, they are a restriction."
Mr George believes that the Covid passes neither limit the spread of the virus nor increase uptake of the vaccine, "so have no place on the statute books".
He added: "Labour, with Plaid Cymru’s support, are only keeping them in place so they do not look like their u-turning on one of the most poorly informed policies ever implemented during the age of devolution."
A Welsh Government spokesperson said: “The Covid pass has been introduced as one measure – among many – to help keep businesses open while also helping to control the spread of the virus and protect the NHS.
“The venues where the Covid pass is required have been chosen because they are mostly indoors and they see large numbers of people congregating closely together for prolonged periods of time. As we know, the longer large numbers of people are close together, especially indoors, the greater the risk of transmission.
“A stakeholder group were kept informed of plans and decisions. We continue to support businesses that are required by law to ask for a Covid pass with a range of advice and guidance.”