The UK must address its failure to present a workable solution to the Irish border problem before Brexit negotiations can proceed to the next stage, a key player in the European Parliament has warned.
The parliament's Brexit co-ordinator Guy Verhofstadt said the answer could lie in Northern Ireland remaining in the customs union and single market post-Brexit, even if the rest of the UK leaves those structures.
However, the Belgian MEP stressed the onus was very much on the UK Government to propose an acceptable resolution, insisting the ideas tabled by Whitehall to date were not up to scratch.
Mr Verhofstadt was in Belfast at the start of a two-day fact-finding mission to the island of Ireland. He met the leaders of the region's main political parties at Stormont before heading to the border to hear the concerns of those living on what will be the UK's only land frontier with the EU.
He told the Press Association a "unique solution" was needed to avoid a hard border.
"A unique solution in that there is no resurrection of a hard border in Northern Ireland because that is not in the interests of business in Northern Ireland and Ireland, neither in the interests of the citizens," he said.
"That is also the reason I am here - to hear from the political parties in Northern Ireland how they can find this solution even though we all know this unique solution has to be proposed by the UK side, because the problem is a consequence of the decision of the UK to leave the European Union."
The former Belgian prime minister will spend Thursday in Dublin, where he will meet Taoiseach Leo Varadkar and also address a special joint sitting of Irish parliamentary committees.
Mr Verhofstadt, who is the link man between the EU negotiating team and the European Parliament, voiced scepticism about the UK's position paper on the border post-Brexit, a document that suggested there would be no need for physical customs checks on the frontier even if the UK was outside the customs union.
"For the moment we don't see a workable solution being put forward by the UK Government," he said.
In an interview with RTE Radio One, he added: "There are possibilities, for example you could imagine one proposal on the table that Northern Ireland continued to be part of the customs union and even of the single market.
"The point is it is the UK Government that has to come forward with such unique solutions."
The MEP's visit comes ahead of next month's crucial summit in Brussels when leaders of the EU 27 will decide whether sufficient progress has been made in the first phase of negotiations to progress talks to focus on future trading relations.
The EU has insisted progress has to be made in three key areas: citizens' rights, the size of the UK's exit bill and the shape of the Irish border post-Brexit.
Asked about speculation Prime Minister Theresa May will outline a sum the UK is prepared to pay in a keynote speech on Friday, Mr Verhofstadt said: "Let's have the speech of Mrs May and then we can make an analysis."
He said there remained a "dispute" on the exit bill and that there were "big differences" in terms of both sides' position on citizens' rights.
European MP Diane Dodds joined her party leader Arlene Foster in the Democratic Unionist delegation that met Mr Verhofstadt at Stormont.
Afterwards, Mrs Dodds insisted Northern Ireland could not be treated any differently to the rest of the UK.
"We would not countenance and indeed it would be calamitous for the economy in Northern Ireland if there were barriers to trade with our largest partner which is the rest of the United Kingdom," she said.
Responding to Mr Verhofstadt's claim that the onus for drawing up a solution rested with the UK, Mrs Dodds added: "These talks will only be successful if the two sides are willing and able to talk through a sensible solution to Brexit.
"So therefore soundbites like this ... will not reflect the reality of the solution when it comes."
Sinn Fein's Stormont leader Michelle O'Neill said she had found Mr Verhofstadt very understanding.
"We've made very clear to him today the issues which we need to see - we need to remain in the customs union, we need to have access to the single market, we need to see no return to hard borders and to see citizens have access to the European Court of Justice," she said.
Mrs O'Neill added: "At least he's had the manners to come and talk to people about the implications which is far more than any British minister has done."
SDLP leader Colum Eastwood said there was an "economic and political tsunami" heading for Northern Ireland.
He said Mr Verhofstadt shared his party's position.
"He has the right position," said Mr Eastwood.
"He understands that we need to accommodate the unique circumstances in Northern Ireland, he understands that people in Northern Ireland voted to remain and that has to be respected."
UUP leader Robin Swann said the lack of a powersharing executive at Stormont, due to an ongoing political stalemate between the DUP and Sinn Fein, was harming the region at a crucial time.
"It was shame and disgrace today that we were meeting as individual parties and there wasn't an executive representing Northern Ireland in what is a crucial moment and crucial activity in how Northern Ireland progresses at this moment in time," he said.
Former Alliance party justice minister David Ford said their discussions had included how the UK's withdrawal from the EU could impact on cross-border security issues.
Mr Ford said: "We had significant discussions on the potential dangers of the UK moving away from the EU arrangements, the need to remain full members of international bodies such as Europol and Eurojust to ensure that the UK plays a part in setting the regulations."