The UK Home Office’s already burdened immigration services could be overwhelmed this summer when a new visa-waiver system comes into force for European business travellers and tourists in April, a leading law firm has said.

There have also been fresh warnings that the electronic travel authorisation (ETA) requirements could threaten the post-peace tourism sector in Northern Ireland, with Americans and Europeans travelling to Dublin and beyond deciding not to bother crossing the border because of the red tape.

The ETA scheme, similar to the electronic system for travel authorisation system in the US, requires visitors who do not need a visa to enter Britain to acquire pre-travel authorisation online at a cost of £10.

It was introduced for passengers from Gulf states last year and from Wednesday was expanded to include citizens of about 50 more countries and territories including Argentina, Brazil, New Zealand, Japan, South Korea and Caribbean nations.

Nicolas Rollason, an immigration partner at the London law firm Kingsley Napley, said the real problems may not emerge until April, when the ETA applies to those travelling by air, Eurostar or ferry.

“The Home Office expects a vast number, 30 million, to apply for ETAs. If even a small portion of these, like 1%, get pushed into the visa system it is just going to overwhelm them. The biggest travel groups are from the US and Europe and this has the potential to create huge queues and backlogs in the visa system. It could be a disaster and cause all sort of knock-on problems,” said Rollason.

Rollason, who also chairs the International Bar Association’s immigration and nationality committee, said it could bring Home Office policy into “sharp focus” if businesses and high-profile artists who had been travelling in and out of Britain for years were confronted with visas for short working stays.

Under the rules, anyone who has a criminal offence that resulted in a prison sentence of 12 months or more, even if suspended, must apply for a visa. This can include long-spent convictions for anything from shoplifting to a drink-driving conviction 30 years ago.

“If you had a conviction for a serious crime 30 years ago, it is mandatory to disclose it. It is ridiculous, you could be rehabilitated but if you are foreigner you simply won’t be able to demonstrate that. The UK operates the harshest rules in the world and they are really unfair on people who had purged a conviction or might have done something in their youth,” Rollason said.

Border Force representatives, airlines and Eurotunnel said there was no evidence of any disruption so far, though there was anecdotal evidence of some passengers in the US arriving at airports without knowing they had to have an ETA to visit the UK.

High-profile sports events in Northern Ireland, including this summer’s Open Championship golf tournament, could be hit by the requirements, as well as many tourist attractions such as the Titanic Experience in Belfast, said Stormont’s economy minister, Conor Murphy.

He said the ETA was “a kick in the teeth” and he had raised the issue in a recent meeting with the UK migration minister, Seema Malhotra.

European citizens ordinarily resident in the Republic of Ireland have been exempted from the scheme as long as they are travelling within the common travel area.

Murphy believes that should also apply to tourists innocently crossing the border.

A government spokesperson said the ETA would “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”.

It said it was committed to working closely with a wide range of stakeholders to ensure the ETA requirement was communicated effectively and to mitigate against it being seen as a barrier to cross-border tourism on Ireland.