Any temporary post-Brexit customs deal must be as close to the status quo as possible, leading business lobbyists have warned.
The British Irish Chamber of Commerce said it was cautious about the feasibility of some proposals being put forward in London on simplifying new trade barriers with Europe.
John McGrane, director of the association, said the ideas were a constructive contribution to the negotiations.
"The recognition of the need for an interim period to allow for the adoption of new systems and infrastructures will be welcomed by businesses who have long warned of the dangers of leaving the Customs Union without such a transition phase," he said.
"It is important that this phase is as close to the status quo as possible with minimum disruption caused by tariff and non-tariff barriers."
One plan being put forward by London is a time-limited transition which would ensure businesses trading in and out of the UK would only have to adapt once to the rule changes.
A two-year period is being suggested by Brexit Secretary David Davis and he is pushing for the UK to have the right to negotiate new trade deals with other countries.
On customs, one option being put forward would see the UK manage a new trade border with administration streamlined to what has been described as the "fullest extent possible".
Mr Davis will also float plans for a customs partnership with the EU which would negate the need for customs checks between the UK and the European bloc.
Mr McGrane said the idea of an alternative customs arrangement was interesting but completely unproven and that anything less than a completely open border is problematic for business and citizens.
The next Brexit negotiation round starts at the end of the month.
Mr McGrane said a comprehensive post-Brexit customs deal is vital for borderless trade on the island of Ireland and between the UK and the EU.
"The future customs relationship that is agreed between the UK and the EU will be hugely important for the future trade relationship on the island of Ireland," he said.
"An agreement must be sought that ensures borderless trade can continue on the island in order to protect the economic and social gains that have been won over the past 19 years with the development of the all-island economy."
Chambers Ireland chief executive Ian Talbot said it was wildly optimistic to think new customs rules could be agreed and in place in less than two years.
"We remain of the view that the best kind of transitional agreement for businesses in the UK and the EU would be the maintenance of the status quo during that period, allowing continued access to the single market and the existing customs union on an interim basis," he said.