BREXIT could affect the treatment of children with heart disease, it has been claimed.
Paediatric cardiologist Dr Brian Craig warned a hard border risked the swift transfer of seriously ill young people.
Existing open border arrangements provide benefits including access to specialist medical services and highly trained clinicians.
The BMA has warned this access is at risk if border restrictions are introduced following the UK's withdrawal from the EU.
Examples of cross-border cooperation between north and Republic include the all-island paediatric cardiology service at Our Lady's Children's Hospital in Dublin.
This enables children from Northern Ireland to receive life-changing heart surgery without having to undertake lengthy journeys or travel to England.
Read More: Brexit could be delayed, warns Jeremy Hunt
The service was introduced following the closure of children's heart surgery services at the Royal Victoria Hospital in 2015.
Now, Dr Craig, who worked for more than 30 years at the Royal, warned a hard border "could risk the ability to transfer children smoothly between Belfast and Dublin".
He was speaking after the Children's Heartbeat Trust (CHT) submitted evidence to a Northern Ireland Affairs Committee inquiry into the implications of the EU withdrawal agreement and backstop.
Between 5-8 children from Northern Ireland will have to access emergency treatment in Dublin each year, CHT said.
"Any impediment on this journey for these critically ill babies and children due to border stops, queues or any other unnecessary delays will have serious consequences for these very vulnerable patients," the trust said.
Dr Craig said recruitment and retention of clinical staff was already a serious problem in the NHS.
"This has been aggravated by the UK's upcoming departure from the EU," he told the BBC.
"This problem is even more acute in specialist areas like paediatric cardiology where we rely on specially-trained practitioners."
The Department of Health said it did not envisage any major disruption to cross-border services.
