A Covid vaccine trial volunteer says he faces cancelling plans to celebrate his 70th birthday in France because he has been "disadvantaged" by taking part.
Tom Williams from north Wales said he was denied an approved jab after having the yet-to-be approved Novavax vaccine.
The UK is on the French amber list, and only those fully vaccinated with an approved jab can travel for holidays.
The UK government's Department of Health said it would ensure volunteers were not disadvantaged.
Mr Williams, who lives in Denbigh, wants people who have taken part in trials to have "some sort of leeway in terms of vaccine certification".
"The World Health Organization should really get their act together, recognise the fact that without volunteers on these research projects you wouldn't get the vaccines in the first place," he said.
"They shouldn't be disadvantaged in any way at the end of the day."
The WHO recommends that "essential travel should be prioritised and facilitated, and that certain measures should continue to apply to reduce the risk of spread of Covid-19 through international travel".
"The British government and the Welsh governments have been talking about vaccine passports now for a number of months," Mr Williams added.
"They must have foreseen that this was going to happen at some stage. It's just a frustrating situation."
The French government only recognises the vaccines approved by the European Medical Agency (EMA) - Pfizer-BioNtech, Moderna, Oxford/AstraZeneca, and Johnson & Johnson.
Novavax is currently being assessed by the UK's Medicines and Healthcare products Regulatory Agency, and is subject to an EMA rolling review. The company said it expected to file for authorisation in the third quarter of this year.
People travelling abroad from Wales and Scotland can show they have been vaccinated using a certificate available online, and those in England have an NHS app.
'Happy to be a guinea pig'
Mr Williams said he decided to volunteer after seeing an advert looking for volunteers for the trial in Wrexham: "I thought, you know, if I can help the [Covid pandemic] situation I'll join.
"I basically wanted to try and get my life back on track and everybody else's life back on track... I was more than happy to be a guinea pig."
Following medical tests, he was given his first jab on 4 November and the second on 25 November. At the time he did not know if he was getting the vaccine or a fake placebo test jab.
Then, in February, when people of his age group were being offered approved vaccines, he was offered a jab.
He contacted the Novavax research centre which told him he had received two doses of Novavax.
"It was only in the last four to five weeks that I started realising that they're going to start issuing vaccine passports or certificates, so I started making some inquiries," he said.
He said NHS Wales told him he would get a vaccine certificate in due course - but that does not help him with his holiday plans.
"Unfortunately because the vaccine is not EMA approved it doesn't hold water if I decided to travel and I want to use the vaccine passport," he said.
He said he had been told by NHS Wales it was not recommended for him to get an authorised vaccine on top of the Novavax: "They obviously don't know what the effect that might have."
Earlier this week he went to a vaccination centre in Bangor, but was again refused.
Mr Williams said if he was unable to go to France with his family on 25 July he would be "very annoyed indeed".
He added: "I appreciate that the UK can't control other countries, but surely this is a worldwide issue.
"Other vaccines are obviously going through similar trials and if none of them are going to be recognised obviously people are going to be very reluctant in future to sign up for any of the trials."
On 11 June, an open letter was published by deputy chief medical officer Prof Jonathan Van-Tam to UK residents who have volunteered to take part in Covid-19 vaccine studies.
In it, he said trial volunteers were "creating great benefit for our society and indeed the rest of the world" and would "not be disadvantaged as global travel resumes".
He added: "We believe the added risk of allowing the relatively small number of clinical trial participants to travel (in the UK approximately 40,000 people, the vast majority of whom have been vaccinated) is strongly outweighed by the benefit of ensuring that recruitment and retention into clinical trials can continue."
He acknowledged "individual countries control their own policies and exemptions" but said the UK government would "work hard to influence this situation internationally".
NHS data shows 505,230 people in the UK have signed up to the NHS Covid-19 Vaccine Research Registry to hear more about volunteering for vaccine trials, and 15,580 of those live in Wales.
Over 15,000 people in the UK took part on the Novavax phase 3 trial, according to National Institute for Health Research (NIHR), the UK government's medical research body.
WHO said there were 287 vaccine candidates in development, of which 102 were in clinical development.
A Welsh government spokeswoman said it was "grateful" to trail volunteers who had "made a significant contribution to the research effort".
She added: "Vaccines not yet licenced but which have been given as part of a phase 3 trial are being recorded on national vaccination systems to ensure vaccination records are complete."
She said the Welsh government's "strong advice" was to "only travel overseas if it is absolutely essential".
A spokesman for the UK government's Department of Health and Social Care said: "The government has committed to take any action available to ensure no volunteers in formally approved Covid-19 vaccine trials in the UK are disadvantaged and everyone who has taken part in a clinical trial can call 119 and get a letter to confirm their vaccine certification."
He said the NHS was urgently working on a technical upgrade to the NHS App to allow residents in England who have taken part in a clinical trial to demonstrate their Covid vaccine status.
What is Novavax?
Developed by US firm Novavax, the vaccine is given in two doses.
It was shown to be 89.3% effective at preventing Covid-19 in participants in its Phase 3 clinical trial in the UK, and about 86% effective at protecting against the Alpha variant.
Like the Oxford-AstraZeneca vaccine, the Novavax jab can be stored at regular fridge temperature, which means it can be distributed more easily.
The UK has secured 60 million doses of the vaccine.
Novavax said: "We firmly believe that clinical trial participants should not be disadvantaged in any way.
"The National Institute for Health Research did confirm to vaccine trial participants that they have the same status as people who have received a deployed vaccine
"We are committed to continuing to work with urgency to achieve approval for our vaccine through MHRA and EMA, and expect to file for authorization in the third quarter of this year."
Responding to Mr Williams' concerns, the NIHR said it was aware of the issue "at the highest levels", and was "working on solutions as quickly as we can".
"We are really grateful to everyone who has taken part in these critical trials and do not want anyone to be at a disadvantage," said a NIHR official.
"We recognise how much vaccine volunteers have already contributed to society and do not want them to miss out on any benefits or feel anxious."