Ireland’s leading Brexit negotiator, Rory Montgomery, said Wednesday the final status of the Irish-UK border remains in question despite assurances from British Prime Minister Theresa May that her government would not impose a hard border between Northern Ireland and the south.

Speaking in Dublin, Montgomery acknowledged that London had given prior guarantees that no trade or security obstacles will exist on the island, but said that following through on such pledges will be “extremely challenging” as May’s parliamentary majority depends on the pro-Brexit Democratic Unionist Party (DUP).

The DUP has made it clear that it will not accept a deal that will “separate Northern Ireland economically or politically from the rest of the United Kingdom”.

May has already publicly committed to having Northern Ireland continue to operate under EU norms and regulations, even if it failed to conclude a trade agreement with the EU that provides full access to the Single Market.

Montgomery is a senior Department of Foreign Affairs official, who currently serves as second secretary general at the department and oversees official co-ordination on Brexit talks.

Montgomery warned in November that London should take notice of the EU paper indicating that the North may have to stay within the Single Market and the Customs Union. Ireland’s Taoiseach Leo Varadkar warned London that trade talks would not begin until the British government submitted a written guarantee that no hard border would come into existence.