People living in the Republic of Ireland have been warned to stop booking Covid-19 vaccination jabs at centres north of the border unless they are registered with a doctor there or have an NHS number.
Hundreds of people from south of the border, some travelling from as far as Dublin and Galway, have been turned away from centres across Northern Ireland in recent days.
The online vaccination booking system does not require applicants to give their home address or medical card number to book a slot, but those details must be provided on arrival, as well as photographic ID.
Some recent media reports suggested that anyone living in the Republic of Ireland can book jabs across the border, but that is not the case.
People who cross the border to work in the healthcare system in Northern Ireland have been vaccinated because they qualified as a result of their job.
However, others are not eligible unless they are registered with a GP in Northern Ireland and have a Health and Social Care (HSC) medical number, the equivalent of an NHS number.
Those who book a slot and arrive to be told they are not eligible do not face any penalties, but are refused a vaccine.
Patricia Donnelly, head of the vaccination programme in Northern Ireland, urged those living south of the border to stop booking vaccinations they are not eligible for.
"Our scheme is not open to the residents of the Republic, sadly, but we are seeing increasing numbers of ineligible people, including people from the Republic of Ireland, trying to book an appointment at one of our vaccination centres," she explained.
"But when they turn up at vaccination centres, they find they're turned away because you would need to have a GP in Northern Ireland, have a health and social care number, or otherwise be eligible for health and social care services in Northern Ireland.
"Unfortunately, it's hundreds. We had trickles of individuals really since the start of the programme, but we've noticed in the last week that it's now turning into hundreds, and that would be the case across every single health trust, they're getting hundreds of people booking on to the system.
"Only those who meet the criteria will be vaccinated. If you turn up and are not eligible you will be turned away and you will have wasted staff time in the process.
"It's difficult when individuals travel some distance and arrive at a vaccination centre, full of hope, that you know here's the beginning of the end for them, and they can get vaccinated.
"They are always turned away, so that is disappointing for them, and a difficult journey for them then to live with that disappointment."
While it is not mandatory for those seeking to book a jab to include their home address, guidance is now being added to the system warning that applicants must live in Northern Ireland or fulfill the other eligibility criteria.
The problem has been caused by the pace of the vaccination programme in Northern Ireland.
While the proportion of second doses is significantly lower than in the Republic, the number of people who have received a first dose in Northern Ireland is now more than 687,500.
Based on relative population sizes, that is equivalent to almost 1.8 million south of the border, where the most recent figure for the number of first doses administered is 492,106.
The pace of the programme north of the border will accelerate further next week.
A huge new mass vaccination centre will be opened at the SSE Arena, home of the Belfast Giants ice hockey team, on Monday.
The centre will have the capacity to vaccinate a minimum of 4,000 people per day, rising to 8,000.
Community pharmacists across Belfast will also join the vaccination programme next week, enabling jabs to be administered from premises on local high streets.
A team of 100 British military medics is also due to arrive in Northern Ireland to help administer vaccines.
The same rules apply to all those seeking to book a slot. They must:
- have Northern Ireland HSC number
- be registered with a Northern Ireland GP
- produce photographic ID.