Ireland’s prime minister has suggested that Britain could strike a Norway-style deal with the EU – forging a bespoke customs union with Europe and joining the European Free Trade Association (Efta).

In his first visit as Taoiseach to Belfast, Leo Varadkar hit out at the advocates of a hard Brexit and said their plans for border controls would throw up a trade border “across Ireland”.

He also said promoters of such a way forward had failed to come up with detailed proposals in the 14 months since the EU referendum last year – and that he believed they would never be able to do so.

The Efta includes Norway, Switzerland, Iceland and Liechtenstein – and previously included Britain, before it joined the EU’s predecessor, the EEC, in 1973.

Efta’s members adopt nearly all EU legislation and standards so they can trade with the bloc, but with exceptions in certain areas, such as agriculture and fisheries. Downing Street has not yet ruled out Efta membership.

“There are people who do want a border, a trade border between the United Kingdom and the European Union and therefore between Ireland and Britain and therefore across Ireland,” Mr Varadkar said in a speech at Belfast’s Queen’s University on the future relations of northern and southern Ireland.

“These are the advocates of the so-called hard Brexit. I believe the onus is on them to come up with proposals for such a border and to convince us and convince you: citizens, students, academics, farmers, business people, civil society, that such a border is in your interest and that such a border would not be a barrier to trade and commerce.

“They have already had 14 months to do so, which should have been ample time to come up with detailed proposals. If they cannot, and I believe they cannot, then we can start to talk meaningfully about solutions that might work for all of us.”

Theresa May has said Britain will leave the single market and EU customs union (AP)

Turning to the subject of trade, he continued: “If the United Kingdom doesn’t want to stay in the Customs Union, perhaps there can be an EU-UK customs union instead. After all the EU has a customs union with Turkey, surely therefore it’s possible to have one with the United Kingdom?

“If the United Kingdom doesn’t want to stay in the single market perhaps it could enter into a deep free trade agreement with the European Union and rejoin Efta, of which it was a member prior to accession, or the European Economic Area.”

The Irish PM, who took office in June, also suggested a “long” transition period where Britain remained in the single market so that future long-term arrangements could be worked out.

He said: “If these things cannot be agreed now, then perhaps we can have a long transition period during which the United Kingdom stays in the single market and the customs union while we work all of these things out.”

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Theresa May ruled out staying in the single market and EU customs union in her Lancaster House speech at the start of 2017.

Efta members, except Switzerland, are also all separately members of the European Economic Area, except Switzerland, which effectively participates in the area through a series of bespoke treaties.

All the Efta member states are members of the Schengen borderless area, except for four remote self-governing areas of Norway, including the arctic archipelago Svalbard.