Avoiding a hard border with Nothern Ireland will become very difficult should the UK change its customs rules after a no-deal Brexit, the Taoiseach has said.
Leo Varadkar explained that if the UK leaves the EU without a deal, it would still be aligned to EU customs and regulations as things currently stand.
He said problems would only arise if the UK decided to make changes.
"In the event of a no deal Brexit - I am nearly always loathe to speculate on because it is speculation, and a lot of it raises more questions than I can give answers. If the UK crashed out of the European Union at the end of March they would still be aligned on customs and regulations," he said.
"So the problem would only arise if they decided in some way to change their customs and regulations - and that's where it could get difficult.
"But that is something obviously we are going to have to talk to them about in a no-deal scenario. There is a real understanding across the EU that this isn't a typical border, that this is a border that goes through villages, goes through farms, goes through businesses and of course is a border that people fought and killed other people over," he said.
To speak of planning for a return of a border with Northern Ireland could easily make that happen, Mr Varadkar has warned.
Mr Varadkar restated his position that the return of a border is not being contemplated.
"We are not preparing for a hard border between Northern Ireland and Ireland. We have made no preparations whatsoever for physical infrastructure or anything like that. We certainly do not want it to become a self-fulfilling prophecy,” he said.
“We simply cannot countenance that being the case. As I say, the whole principle of avoiding a hard border can only be achieved with a deal and its customs divergence which would create the problem,” he said.
He said the best way to avoid a hard border is to have a deal, and there is a deal on the table. “I hope Westminster and the European Parliament will be able to ratify it in the New Year. And we will continue to work with our European partners and the UK and our European guarantees to give assurances and guarantees on that issue, obviously without reopening the Withdrawal Agreement itself,” he said.
Mr Varadkar remained adamant about the border issue but added there had already been some talks with the UK and Ireland's European partners about difficulties Ireland could face if the UK changed their customs regulations.
"The answer I've been giving people all along is the honest truth," he reiterated.
The Government unveiled contingency plans to cope with a potential no-deal Brexit on Wednesday, identifying affected sectors that would require up to between 40 and 50 pieces of new legislation.
The no-deal plans include the purchase of land at ports to prevent congestion from new customs.
"The contingency plans we are making are very real but they are happening at our ports, particularly Dublin Port, Dublin Airport and Rosslare, where we have acquired or are in the process of acquiring land and will develop border control posts in those places," Mr Varadkar added.
"They may be needed in March in the event of a hard Brexit, but if they are not needed in March, they will be, or are very likely to be, needed at some point into the future.
"So that is the contingency planning that we are making, but we are not making (plans for) the border between Northern Ireland and Ireland.