Prime Minister Theresa May's talk of a frictionless Irish border after Brexit is "nice words", according to a customs expert.
Michael Lux, the former head of the European Commission's customs procedures, has been giving evidence to the Northern Ireland Affairs Committee.
He told MPs: "If Northern Ireland is not part of the EU customs territory then there is a customs border.
"I think what [Mrs May] meant was to keep the burden as small as possible."
Earlier this month, the prime minister said Brexit will mean the UK leaves the European Customs Union.
Independent unionist MP Lady Sylvia Hermon asked Mr Lux if Mrs May could achieve "a seamless border".
He replied: "If you define seamless as no border controls then the answer is no, at least for Ireland because it is obliged to apply EU law.
"If the UK is outside the EU it can do whatever it likes."
But he added that "there will be a lot of fraud going on" if the UK does not implement some form of border measures.
There were gasps from some MPs when Mr Lux gave another example of problems that would arise post-Brexit.
"If a Northern Ireland person has a walk and takes its dog over the border there are specific rules on what kind of document you have to have," he told the committee.
"You need a specific document - if you bring a horse riding you also need a specific document."
Lady Sylvia responded: "We have a significant number of farms straddling the border.
"There will be lots of dogs on farms. I cannot imagine a form has to be filled out when a dog runs from the Republic of Ireland to Northern Ireland.
"This is just unenforceable."
Mr Lux said he had suggested the EU allows Northern Ireland to remain in the customs union during a "transitional phase".
He believes this could be for a period of up to 10 years.
He added there was something of a precedent around German re-unification, when the former East Germany was part of the customs territory while still outside the EU.