THE continuing controversy over the Irish border has become the biggest threat to the Brexit talks as politicians from the UK and Ireland raise the stakes on the issue.

After Phil Hogan, Ireland’s European Commissioner, made clear Dublin would “continue to play tough to the end” over its threat to veto the UK Government’s hoped-for trade talks until it had firm guarantees from London over the border issue, Liam Fox intervened to suggest there could be no deal on the Irish border until there was an “end state” on Britain’s future trading relationship with the European Union.

Mr Hogan had argued it was a “very simple fact” that the UK deciding to stay inside the single market and customs union, or allowing Northern Ireland to do so, would end the stand-off.

But the International Trade Secretary hit back, saying: "We don't want there to be a hard border but the UK is going to be leaving the customs union and the single market."

He told Sky News there had always been exceptions for Ireland such as on voting and residence rights and that a “certain asymmetry” would have to be part of an agreement with the EU.

“But we can't come to a final answer to the Irish question until we get an idea of the end state and until we get into discussions with the EU on the end state that will be very difficult, so the quicker that we can do that the better and we are still in a position where the EU doesn't want to do that."

Moving onto phase two of the negotiations can only happen if all 27 leaders of the remaining EU countries agree "sufficient progress" has been made on the first set of issues.

Irish MEP Mairead McGuinness, a member of Taoiseach Leo Varadkar's Fine Gael party, said she was "troubled" by Dr Fox's comments, noting: "I hope the UK is not holding the Irish situation to ransom in these negotiations, it is far too serious and far too critical."

Arlene Foster, leader of the Democratic Unionist Party, has been adamant that the UK will act as one on the issue of Brexit and told her party’s conference at the weekend: "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."

As tensions continue to mount, Ms Foster also accused Simon Coveney, the Irish Foreign Minister of “aggressive” behaviour, asking why he had chosen “this moment in time to talk about his aspiration for a united Ireland in his political lifetime?"

Meanwhile, John Bruton, the former Taoiseach, accused the UK Government of jeopardising the Good Friday Agreement, which underpins Northern Ireland’s peace process. He said: “The UK Government did not think of Ireland at all when it was making this decision[on Brexit].”

Helen McEntee, Ireland’s European Affairs Minister, noted: "We cannot say that there is sufficient progress on the Irish issue when there has not been.”

She told Channel Four News Mrs May's commitment to no return to a hard border on the island of Ireland was welcome.

"However, if all of the options that we feel can make that possible have been taken off the table, then we need them to produce something else that will give us confidence, moving into phase two; that this can actually be achieved. To date this has not happened.”

She added: "But, obviously, I would hope, we need to be optimistic, that negotiations can continue before the December council meeting and that we can see some solution to this."

Theresa May has been given until December 4 by Brussels to make more concessions. On that date, the Prime Minister is due to have another dinner with Jean-Claude Juncker, the European Commission President, which will take place just 10 days before the crunch December European Council.