The Executive in Northern Ireland must "be honest" with people on the efficacy of face coverings, a top Oxford academic has said.
Professor Carl Heneghan believes the evidence used to impose the mandatory wearing of masks in public spaces is "poor quality".
The clinical epidemiologist, who made headlines after uncovering a discrepancy with how Covid-19 deaths were recorded in England and Wales, said Northern Ireland should not have followed other UK regions by introducing the policy.
Beginning on Monday, people must wear a face covering while in retail stores and some other enclosed spaces, unless exempted by certain conditions.
Anyone who fails to comply risks a £60 fine - reduced to £30 if paid within 14 days - although it is unclear how, if at all, the regulations will be enforced.
The Department of Health said responsibility for imposing penalties rests with the PSNI, but it said "primary responsibility" lies with traders. Retail NI, meanwhile, has insisted it is not for shop owners or staff to enforce the rules.
Professor Heneghan, who is director of the University of Oxford's Centre for Evidence-Based Medicine, said the implementation of the new regulations has been a "triumph of opinion and policy over evidence" - one made on observational data, and not on randomised, controlled trials.
The Department of Health (DoH) has insisted, however, that it considered a "wide range of evidence" in making its recommendation for masks to be made mandatory, including two Royal Society reports.
Professor Heneghan said: "Northern Ireland should be honest with the population when it deems to put a whole policy together and say that we're going to do this, and be clear: there is very poor quality evidence to inform what we're going to do next, but we're going to do it anyhow."
The GP insisted the policy has been driven on the basis of decision-makers selecting evidence to suit their opinions.
"There's a long history of people trying to use observational study to confer treatment effects. And that's what people have done," he said.
"But there's a real problem with when you do that approach, because you can cherry pick the evidence to suit your argument.
"So for instance, people use observational data that in countries with masks, infection rates are low.
"However, when you come to higher quality evidence, randomised, control trials, you see that's not the case.
"The question is: if you keep introducing interventions like this, people are not questioning do they work, and not questioning how will we evaluate if they work.
"This is a triumph of opinion and policy over evidence. And the biggest factor is, how can you inform the public of the benefits and the harms of these interventions?"
Professor Heneghan said not enough focus has been given to the potential counterbalance - or equipoise - to public health intervention policies such as masks.
He said: "The other thing people don't understand is that interventions are likely to increase benefit as well as increase harms. People seem to perceive there are no harms with any interventions, which is wrong.
"It is not clear to me that cloth masks are beneficial or harmful, but it is clear that we don't use them in healthcare settings because they have been shown to increase the rates of infection compared to surgical masks.
"By all means wear a mask, but if you're imposing it on society, then politicians should really ask questions.
"It's unacceptable for people to say: 'Here's a review on poor quality evidence that we're going to use to confer a policy'."
Addressing how it is presented to the public, the Oxford expert added: "Anything that has 'might', 'could' or 'may' shows you that you have a problem.
"If people want to wear masks, then that's completely up to them. My job is to reduce the uncertainty about whether it's beneficial or harmful. If you wear it, unfortunately there's no evidence that I can help you with that's going to benefit you with high quality studies."
In mid-July, Prof Heneghan spotted major discrepancies in how Public Health England recorded coronavirus deaths that led Health Secretary Matt Hancock to order an urgent review.
A health spokesperson said: "The department has considered a wide range of scientific evidence from a number of sources when developing recommendations around the use of face coverings and new evidence emerges all the time."
Questions have also been raised about the huge disparity in cross-border penalties.
The £60 fine here is far below the Republic, where anyone caught flouting the rule can face a fine of up €2,500 or up to six months' imprisonment.
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