The Brexit agreement between the UK and EU prioritised avoiding the need for checkpoints on the Irish border.

Tour operator Caroline McComb said the “seamless and painless” cross-border journey was vital for the tourism industry in Northern Ireland.

The director of McComb’s Coach Travel said ETAs would create an “obstacle” for visitors – particularly those who want to visit Northern Ireland for a short time.

She said: “If you have a group of 50 people who are hiring a coach suddenly you’ve £500 added to your group’s cost.

“And that will make people think if I’m travelling from Dublin to Donegal, is it really worth going via the Causeway Coast and stopping at the Giant’s Causeway?”

A UK government spokesperson said ETAs will deliver a “more streamlined, digital immigration system which will be quicker and more secure for the millions of people who pass through the UK border each year”.

The government also said it was committed to working with the industry and others “to ensure the ETA requirement is communicated effectively and to mitigate against it being seen as a barrier to cross-border tourism in Northern Ireland”.

Travellers are being advised to apply for an ETA at least three working days in advance of arrival.

But sources have indicated that many applications are decided within minutes, which would allow for the continued possibility of spontaneous cross-border trips.

Ministers and officials in the devolved government in Belfast are expected to continue lobbying London for an exemption ahead of the final phase of the ETA scheme being implemented in just under three months’ time.