There have been fears that a return to a physical border in Ireland would be a catalyst for a return to sectarian violence.

But there has also been speculation that the UK's determination to set its own trade rules would require customs checks at the border with the EU.

Despite its reaffirming, in another position paper on Tuesday, that it intends to leave Europe's customs union and strike new trade deals with the rest of the world, the UK government believes it is possible to maintain a "seamless and frictionless" border in Ireland.

The government recognises there is no precedent for what it wants to achieve.

In a media release a UK government source said "both sides needs to show flexibility and imagination when it comes to the border issue in Northern Ireland."

"As we shape the unprecedented model, we have some very clear principles. Top of our list is to agree upfront no physical border infrastructure ??? that would mean a return to the border posts of the past and is completely unacceptable to the UK.

"Our paper sets out some creative options on customs and shows the priority we place on making progress on this."

One of the UK's desired options for a future trade relationship with the EU would be a 'customs partnership' that would mean no need for customs checks, it said.

The other option, which it described as 'highly-streamlined customs arrangements', would avoid the need for customs checks using bureaucratic and technological solutions.

According to the media release, this would include a waiver on entry/exit declarations and continued membership of the Common Transit Convention to help Northern Ireland and Irish companies transit goods.

It would reflect the "unique circumstances of Northern Ireland, such as new trusted trader arrangements to deliver highly streamlined processes for larger traders and cross-border trade exemption meaning no new customs processes at all for smaller traders."

Earlier this week Ireland's EU commissioner Phil Hogan told the Financial Times that Dublin could block a customs deal unless the problem of the Northern Ireland border was satisfactorily solved.

"There's a high level of delusion in London at the moment about what is required to be done," Mr Hogan said.

The UK also aims to protect the free movement Common Travel Area with Ireland, and associated rights for UK and Irish citizens, and uphold the Belfast Agreement.

The paper dismisses an idea which had been mooted in Ireland - especially among republicans - that the UK's customs border could be placed in the Irish Sea rather than at the Irish border.

It says this is not "constitutionally or economically viable".

But the UK's border model was attacked as a "smugglers charter" by Irish Senator Mark Daly, the deputy leader of the Fianna F??il party.

He told the BBC that after the UK left the customs union and signed other trade deals there would be a trade differential on goods between the UK and EU.

That would make the Irish border a "backdoor into Europe" for people smuggling goods that were cheaper in the UK, he said.

"We all agree on wanting a seamless and frictionless border but it's the practicalities [that matter]."

Around 30,000 people cross the border every day for work, school or other reasons, he said. Without immigration checks the border could also be used by people trying to get into the UK illegally.

He agreed that a similar frictionless border model worked between Sweden and Norway, which is not an EU member.

"That would be a practical model [for Ireland] if we didn't have the history we have on this island," Senator Daly said.

For example, he said, hardliners on either side - including him - would oppose having foreign customs officers based on their territory, which would be necessary to make it work.