From Wednesday, travellers from the UK aged 12 and above are only permitted to enter Malta if they have had both doses of a coronavirus vaccine.
But authorities in the central Mediterranean archipelago revealed on Monday they will only accept printed letters sent by the NHS as proof.
That means tourists planning to use the NHS app to demonstrate their status face being turned away at UK airports or the border in Malta, even if they are fully vaccinated.
The UK Government’s website states that letters are expected to take “up to five working days” to be delivered.
Several affected people have sent Twitter messages to the British High Commission for Malta stating the policy means their trips cannot go ahead as planned.
Mark Holland, from Hove in East Sussex, wrote: “I travel in under three days and have no time to request a letter.”
Children aged five to 11 can travel if they are accompanying their fully vaccinated parents or legal guardian, and must show evidence of a negative PCR test taken within 72 hours before arrival.
No test is required for those aged under five.
The requirement for UK arrivals for those aged 12 to 17 to be fully vaccinated is effectively an outright ban, as the UK is not vaccinating under-18s.
Katie Crookshank, of London, wrote to the High Commission: “We have a 12-year-old girl who is distraught as she now can’t be a bridesmaid in August.”
She continued: “The reputation of Malta being a family friendly island is being damaged. Why can’t they be PCR tested?”
British High Commissioner to Malta, Cathy Ward, replied that she was “so sorry to hear this”.
She added that the Maltese authorities have said “the guidance on teenagers is due to the virus now spreading fastest in this age group and they are worried about the spread of the Delta variant”.
There was a surge in demand for holidays in Malta, Spain’s Balearic Islands, Portugal’s Madeira and a number of Caribbean destinations after both the UK and Scottish governments announced last week they would be moved to the green travel list on Wednesday at 4am.