Business leaders in the north west yesterday told Jeremy Corbyn that the lack of clarity on the border was testing their patience.
The Labour leader met commercial figures in Londonderry, where he was told it was "vital" senior politicians negotiating the Brexit deal come to the frontier and see its day-to-day realities.
He also visited the Tyrone/Donegal border, where he spoke to traders from Strabane and Lifford.
Mr Corbyn reiterated his call for the return of the Assembly, which he said was essential for peace.
He said: "The future prosperity and peace on these islands also depends on how we deal with Brexit.
"I believe the British Government is making a mess of these negotiations. I said on the very first day after the referendum that we wanted a constructive and trading relationship with Europe in the future.
"The idea of any kind of hard border is impossible to conceive how it will do anything other than seriously damage the economy of this area, which is an area that already has an excessive level of unemployment and desperate levels of poverty.
"We do need a customs union and tariff-free access to a single market and that means trade with no regulatory divergence.
"What we are arguing for is a relationship with the single market based on protecting and improving existing trade standards and rights."
President of Derry Chamber of Trade Jennifer McKeever outlined why 90% of her members would prefer to stay in the EU customs union and single market.
She said: "We are a city region that sits on a border spanning two counties, two countries, two jurisdictions - and we run businesses serving both.
"Every day thousands of us cross the border for work, for education, for health and for trade.
"The EU is not a foreign, faraway bureaucracy - it's a couple of miles from where we sit. It's where many of our staff live and where we find our customers and trading partners.
"Everybody accepts that it would be unacceptable to reintroduce a hard border in Ireland.
"What is infinitely more difficult to understand is how we can pull out of the customs union and single market without one."
She added: "During the past two years businesses here have been hugely resilient and pragmatic and tried to anticipate the challenges and tried to make contingency plans with little or no clarity on what we were planning for.
"To be honest, two years later, our patience is wearing thin."
Bonnie Anley, chair of Foyle Port, said: "We, like many businesses here, feel there is still a lack of clarity in the key areas of trade, travel and customs.
"As a key driver of the north west economy, we are keen that the post-Brexit arrangements are firmed up as soon as possible.
"We encourage Mr Corbyn on his return to London to impress upon the Government and his own party colleagues that it is imperative a swift and positive remedy is found to the problems any sort of hard border would impose."