Whitehall officials conceded that would be the side-effect of maintaining the so-called Commons Travel Area, which allows free movement between the island of Ireland and mainland Britain.
The admission came as a leading Irish politician warned that the Government’s plans not to re-introduce border posts between the Republic of Ireland and Northern Ireland after Brexit would become a “smugglers’ charter”.
Ministers today published its proposals for how the border will operate after the UK quits the European Union, given that the Republic of Ireland will remain a member of the bloc.
The 25-page document says: “The UK can provide a clear assurance that the CTA can continue to operate in the current form nd can do so without compromising in any way Ireland’s ability to honour its obligations as an EU member state.”
It adds: “It is important to note that immigration controls are not, and never have been, solely about the ability to prevent and control entry at the UK’s physical border.
“The UK is confident that it will be able to maintain existing movement to the UK from within the CTA without requiring border controls.”
However, government officials struggled to explain how the UK would prevent EU citizens gaining back door entry to Britain via the Republic.
One said: “Immigration control has never been about physically stopping people at the physical border.
“We’re saying it’s perfectly possible, and we will introduce a system, that allows you to maintain operation of the Common Travel Area whilst having a UK system of immigration control that will also apply in Northern Ireland for EU nationals.”
The Government paper also set out plans to maintain the same food standards arrangements between Ireland and the UK, thereby taking away the need for border customs checks.
But Irish senator Mark Daly, the deputy leader of Fianna Fail, hit out at the Government’s proposals.
Speaking on Radio Four’s Today programme, he said: “While Ireland wants to get the best deal for Britain, it appears Britain doesn’t want to get the best deal for itself.
“It throws out lines about frictionless and seamless borders and hopes that it will get traction in the EU, but hope is not a policy. It appears more like fiction and clueless on this island – it would be a smugglers’ charter.
“There are over 300 miles of border between the north and the south of Ireland, and there are more border crossings on this island than between the European Union and all the countries to the east of it.”
“If there is trade differentials between the UK and other non-European partners, our border would be a back door into Europe, so people would import goods that are cheaper under tariff arrangements with the UK and then bring them into the Republic and then onto the EU.
“The UK wants an advantage economically over the EU and preferential trade agreements with non-EU countries – that becomes a smugglers’ charter on this island.”
However, Northern Ireland Secretary James Brokenshire insisted the Government’s plans were workable.
He said: “If you look at what [EU chief negotiator] Michel Barnier has said and others within the EU, there is a recognition that there will need to be specific arrangements in relation to customs and other elements in terms of creating that frictionless border,” he said.
“I think there is a shared objective that we have, that the EU has and the Irish government has, in finding that solution.”