Revenue has warned of its "fundamental concern" about businesses who have not registered for an important Revenue number which is considered the first step in getting Brexit ready.
Around 15,000 Irish businesses out of an estimated 90,000 which trade with the UK, or whose goods move through the UK, have yet to register for an Economic Operators Registration Identification (EORI) number.
Lynda Slattery, head of Revenue's Brexit Policy Unit, said that engagement with the SME sector has shown that some businesses believe that Brexit "just won't happen" which she described as "extraordinary."
"As soon as the UK leaves the Customs Union, customs formalities apply. That is an absolute fact, that is not going to change," Ms Slattery said.
. It has no impact on the negotiations, customs, could kick in on the 1st January."
Yesterday, the Taoiseach urged small businesses which either export with or import from the UK to register and engage.
Micheál Martin said he was concerned that there was some complacency in the SME sector and he called on them to get organised.
Ms Slattery said Revenue is especially concerned about the readiness of the construction sector.
She also said it is imperative that businesses have the ability to interact with the customs systems.
It is not enough, she said, just to have EORI registration.
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"The EORI registration is the first that opens the door to customs, that literally brings you in the door. You need to take the steps beyond that in terms of having the ability to be able to interact with the custom systems," she added.
January 1 is the legal date of the new regime and there is no indication of there being any grace period or change to that date, Ms Slattery said.