Northern Ireland's membership of the EU customs union after Brexit is "a matter for negotiations", Downing Street has suggested. 

The Government here wants assurances from London that there will be no "hard border" with Northern Ireland, warning that deadlock in Brexit negotiations cannot be broken until the issue is resolved.

With the UK committed to withdrawing from the customs union and the single market, Foreign Affairs and Trade Minister Simon Coveney said it was difficult to see how border checks could be avoided if this resulted in "regulatory divergence".

Taoiseach Leo Varadkar has previously suggested a "bespoke" arrangement similar to that operated on the Isle of Man could be reached.

This arrangement would see Northern Ireland, or the whole of the UK, continue to observe the rules of the single market and customs union without necessarily remaining a member of them. 

But Democratic Unionist Party leader Arlene Foster warned against any divergence between the regulatory framework of Northern Ireland and the British mainland.

"What we don't want to see is any perception that Northern Ireland is in any way different from the rest of the UK, because that will cause us great difficulties in relation to trade. The single market that really matters to us is the single market of the United Kingdom," she said on BBC Radio. 

Asked at a regular Westminster media briefing today whether Northern Ireland could remain in the customs union following Brexit, a Downing Street spokesman said: "That is a matter for negotiations".

"Our position on Northern Ireland has been set out in the papers and we need to continue to negotiate to find an innovative way forward," the spokesman said.

Arriving in Brussels for a gathering of leaders from the EU and former Soviet states, Prime Minister Theresa May said it was time to move on to the "next stage" in the negotiations, including talks on a free trade deal.

But Simon Coveney insisted in Brussels today that EU leaders would not give the green light for the phase two negotiations to begin at their summit in December unless there was progress on the border issue. 

He said British assurances on the issue were "aspirational" and that there had to be a "credible roadmap" from the UK setting out how they would ensure there was no return to a hard border. 

"We can't move to phase two on the basis of aspiration," said Mr Coveney.

"We have move to phase two on the basis of a credible road map or the parameters around which we can design a credible road map to ensure that it doesn't happen," he said. 

"The truth is that if we see regulatory divergence between the two jurisdictions on the island of Ireland it is very hard to see in that scenario how you avoid hard border checks. So we need progress on this issue in the context of the regulatory divergence issues".

"I hope and expect that we can get that by December so that we can all move on. If we can't, then I think there is going to be a difficulty coming up," the Minister said. 

Mr Coveney added that the other member states were fully behind Ireland's stance on the issue.

"I don't think Ireland will have to block anything on its own. There is absolute solidarity across 27 countries here. They are with Ireland on this," he said.

"We are not talking about a 'no deal' here. What we are talking about is whether we can move on to opening up phase two in parallel with phase one issues in December. Without sufficient progress on the Irish issues that can't happen," he stated.