Christoph Leitl, President of Eurochambres (European Chambers of Commerce and Industry), wants the UK to remain tied to the Customs Union once it leaves the single bloc.
Mr Leitl warned that EU countries would need to recruit thousands of extra customs officers to beef up border security if Britain leaves the union.
He said: “If there is no customs union, that will mean that we will need 10,000 extra customs officials in Europe.
“Huge waiting times, huge traffic measures to handle the traffic jams, so to speak. Those who initiated it really did not think things through and act irresponsible in my view.
He added: “I say, stay inside the Customs Union, in the single market.”
The EU and Britain’s current Brexit position was “completely confusing”, Mr Leitl believed, but he was “optimistic that they can find an agreement”.
And with negotiations between Theresa May and the EU negotiating teams reaching deadlock, Mr Leitl admitted the current economic outlook was “insecure”.
But he warned Theresa May that failing to strike a trade deal with the EU would harm British business more than the 27 other EU states’ economies.
“If there is no customs union, we will need 10,000 extra customs officials. I say, stay inside the Customs Union, in the single market.” ~ Christoph Leitl
Mr Leitl said: “Fifty percent of all UK exports go to the markets of the European Union, but only five percent from the markets of the European Union to Great Britain.
“It must be in the vital interest of Great Britain to get out of politics, but to stay in business economically.”
The Austrian president of Eurochambres called for a “transitional regime for he issue of migration that concerns you so much.”
He added: “As always in such negotiations, it’s a poker game till the end, as solutions are being sought.
“Then, at some point the clock is stopped, and a solution is announced.”
Last week’s Brussels summit failed to break the deadlock with Mrs May accused of offering “nothing new”.
EU leaders and the British negotiating team remain divided over as solution to the Northern Ireland backstop to ensure a free-flowing border.
Theresa May has vowed not to accept a deal that keeps Northern Ireland tied to EU customs rules while the rest of the UK leaves.
At last week’s talks, she admitted she could seek an extension of a few months to a December 2020 transition date.
A showdown summit planned for November may now be pushed back to December.