LONDON (Reuters) - Britain will not resolve the question of the Irish border after Brexit until it has also agreed the outline of a trade deal with the European Union, the country’s International Trade Minister Liam Fox said on Sunday.

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The EU has said “sufficient progress” needs to be made on the border between the Republic and Northern Ireland, along with two other key issues, before EU leaders meeting at a summit on Dec. 14-15 can approve the opening of trade talks next year.

However, Fox said it would be very difficult to address the issue of the border while Britain’s relationship with the EU after Brexit remains unclear.

“We don’t want there to be a hard border but the United Kingdom is going to be leaving the customs union and the single market,” he told Sky News.

“We can’t get a final answer to the Irish question until we get an idea of the end state, and until we get into discussions with the European Union on the end state that will be very difficult.”

Dublin wants a written guarantee that there will be no hard border between the Republic of Ireland and Northern Ireland.

Earlier on Sunday Ireland’s EU commissioner said Dublin would “continue to play tough” over its threat to veto talks about trade after Brexit unless Britain provided guarantees over the border between Northern Ireland and the Republic.

Phil Hogan, the EU’s agricultural commissioner, said that Britain, or Northern Ireland at least, should remain in the single market and the customs union to avoid a hard border dividing the island.

“If the UK or Northern Ireland remained in the EU customs union, or better still the single market, there would be no border issue,” he told the Observer newspaper.

Irish and EU officials say the best way to avoid a “hard border” - which could include passport and customs controls - is to keep regulations the same north and south, but the Northern Irish party that is propping up May’s government will oppose any deal that sees the province operate under different regulations to the rest of the United kingdom.

“We will not support any arrangements that create barriers to trade between Northern Ireland and the rest of the United Kingdom or any suggestion that Northern Ireland, unlike the rest of the UK, will have to mirror European regulations,” the Democratic Unionist leader Arlene Foster said on Saturday.

Ruth Davidson, leader of the Conservatives in Scotland, said on Sunday that the Irish border was “one of the really difficult bits” of the negotiations.

She said Britain’s unique future position as the only country that had left the European Union meant its did not need an “off-the-shelf” solution, although she did not specify how the issue should be resolved.

She said any delay in moving onto trade talks would have serious repercussions for businesses.

“I think that it is really important that we get the transitional deal nailed down; that’s not for government, that’s for businesses so they know what they are doing next year and they are able to plan,” she said.

“If we don’t make it through in the next two weeks to move onto that next phase, then we are rapidly going to run out of time in terms of getting us to a good position at the time that transitional deal is supposed to take place.”