A Covid vaccine SLOTXO 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".