Northern Ireland needs a voice at the Brexit talks table, Jeremy Corbyn has said.
The Labour leader visited an Irish border bridge on Friday as part of his two-day tour of the country.
The porous 300-mile frontier is one of the most vexed issues facing negotiators in Brussels but Northern Ireland has no ministers to intervene since devolved government at Stormont collapsed more than a year ago.
Mr Corbyn addressed a meeting of business leaders in Derry: "Please, to the parties in Stormont: you have to come together to re-form a government there.
"It is impossible to go through a period so crucial as Brexit negotiations without a voice for Northern Ireland being made at the table by the political classes in Northern Ireland.
"I hope they understand that message and I hope that we can make very rapid progress on that.
"There is to be a transition period but the transition period is not unlimited, that we well know, and crucial decisions are going to be made in the next three months and I understand very clearly the message that you have given me here this morning."
A "backstop" border option if no other deal was reached with the EU would see Northern Ireland and the rest of the UK follow Brussels regulations relating to co-operation on the island of Ireland to protect frictionless all-island trade.
The British government has said it is focused on securing a customs deal which would avoid the need for such a backstop and has ruled out anything which would create a regulatory difference between Northern Ireland and the rest of the UK.
Mr Corbyn walked the Lifford bridge between the north and Republic, close to the Co Tyrone town of Strabane, and took selfies with passers-by.
He supports a customs arrangement with the EU to protect trade and ensure no regulatory barriers after Brexit.
The British government is examining whether a customs partnership or a technological solution to border checks can be made to work.
The British opposition leader added: "Any kind of border, physical border, virtual reality border, technological border, whatever, would be very damaging to the economy."
Mr Corbyn said a hard frontier would seriously damage Northern Ireland's north-west, including the life chances of those already suffering from excessive unemployment.
He claimed the British government was making a mess of the negotiations and was too divided and weak to get a good Brexit deal.
Stormont has not sat for months following a dispute over a botched green energy scheme.
The north's rudderless public services were thrown into sharp relief recently when a Civil Service decision to approve a major incinerator project in the continued absence of powersharing ministers was overturned by a court.
Mr Corbyn added: "It is unconscionable that you have civil servants making major decisions, then challenged by the courts, with elected politicians having no say whatsoever."
