The transition period for the UK's departure from the EU is too short, a group representing businesses in Ireland and the UK has warned.
British Irish Chamber of Commerce president Eoin O'Neill said talks on the framework for a future relationship would need to be finalised during the transition phase if negotiators wanted to avoid a "cliff-edge scenario".
"Both the conditions of the transition phase and the full withdrawal agreement text will need to be finalised by October this year to allow time for ratification across the EU institutions and in Westminster," said Mr O'Neill.
Therefore, he said, the nature of the future relationship will not be discussed in detail until after the UK has left the EU in March 2019.
"This timeline shows that the clock is indeed ticking, and that we need to see some urgency from the UK Government in putting forward its proposals for its future relationship with the EU," he said.
The chamber's head of Brexit policy Katie Daughen said: "On the other point around the transition phase with it being too short, the EU Commission has set the deadline of December 31 2020 on the basis that it is the end of the currently multi-annual financial framework.
"I think there would be an option to extend the transition phase beyond that but it would require the UK paying into the next framework budget, which could be politically very sensitive within the UK."
The chamber president told an Oireachtas committee that the lack of clarity and certainty over trade was causing concern for small businesses across the UK and Ireland.
"The big problem at the moment is the unknown," Mr O'Neill said.
"In an environment where you don't know what the end looks like, it's very hard to plan for that finish and that is what businesses' biggest concern is.
"Until the UK comes forward with proposals on how it will address the above conundrum, business will remain uncertain about what its future trading environment will ultimately look like."
He added that anger and anti-UK sentiment was developing in Brussels over Brexit.
"It is not possible for the UK to pursue its own trade agenda while maintaining borderless trade with Ireland and therefore the rest of the EU," he said.
"Even if the UK retained full alignment with the EU regulations and standards, some sort of border checks would need to take place in order to collect the appropriate customs and duties on these goods."
Mr O'Neill said the easiest way to move forward was for the UK to remain in both the EU customs union and internal market but if this was not possible, the chamber's Brexit trade plan would be the best plan B.
The chamber, which represents businesses across both islands, recently published a document outlining its solution for trade post-Brexit.
Trade between Ireland and Britain is worth more than 65 billion euro a year and sustains over 400,000 jobs across the islands.