The EU and Republic of Ireland are "intensifying" discussions about the Irish border in the event of a no-deal Brexit, senior EU officials have said.
On Monday, the European Commission said it believed a no-deal outcome was now looking increasingly likely.
The EU said it would expect the UK to "live up to its commitments to avoiding a hard border".
The prime minister has said Northern Ireland is unable to prepare properly for a no-deal Brexit.
Speaking in the House of Commons on Monday, Theresa May said she had wanted to deliver Brexit on 29 March.
"But, I'm conscious of my duties as prime minister to all parts of our United Kingdom and of the damage to that union leaving without a deal could do, when one part of it is without devolved government and unable therefore to prepare properly," she said.
Former Northern Ireland Secretary Owen Paterson said he was surprised to learn that Northern Ireland had not been able to prepare properly.
Democratic Unionist Party (DUP) deputy leader Nigel Dodds accused Mrs May of a "fundamental lack of preparation" and said "the government is entirely responsible for that".
DUP's Nigel Dodds accuses Theresa May of a "fundamental lack of preparation" over #Brexit, saying government has known for "considerable time" the UK was due to leave the EU on 29 March - but still needs an extensionhttps://t.co/5APtAe8Gwp pic.twitter.com/OuaADQYDms— BBC Politics (@BBCPolitics) March 25, 2019
The prime minister said it "would not have been appropriate" for no-deal to go ahead on 29 March because "the Northern Ireland civil service do not have the powers to take necessary decisions in the event of no deal".
Mrs May told MPs a no-deal Brexit would require some "direct application of powers" from Westminster to Northern Ireland.
"If there is no Stormont government, if powers are needed and ministerial direction is needed which is not available to the civil servants currently, it would require some form of direct application of powers here from Westminster," she said.
DUP position unchanged
The DUP and Mrs May talked by phone earlier on Monday.
Mrs May updated the DUP on her latest conversations with Conservative Brexiteers which took place at the prime minister's country residence, Chequers, over the weekend.
The DUP - whose support for Mrs May's deal will be crucial if the government is to win the vote - said its position had not changed after the phone conversation.
On Monday, the EU said that a no-deal Brexit would have a disruptive impact on cross-border trade and supply chains.
"The controls will have to be done where they belong but that doesn't mean we would want to see visible infrastructure at the border," an EU official said.
"We're working very closely with the Irish authorities to try and perform controls away from the border if at all possible."
Talked to @niexecutive source regarding north south agri-food trade after Brexit who expressed more concern about regulations than tariffs - said if all NI produce is regarded as not meeting EU regulations could amount to a "blockade"— Mark Devenport (@markdevenport) March 25, 2019
An EU spokesperson added that if a no-deal Brexit happened "both sides would need to take unilateral and temporary measures to protect legitimate trade, consumers and public health".
"The commission is ready to make additional resources available to Ireland (technical and financial) to address any additional challenges," the spokesperson said.
They added that "in all scenarios, the Good Friday Agreement will continue to apply".
Earlier, International Trade Secretary Liam Fox said he was in "regular contact" with the DUP.
He said the party understood the weight of the decisions it was having to take - especially with regards to the future of the union.
"They understand more than most the complexities of the Irish border issue," he said.
"But there has been a clear commitment to leave the EU, all MPs have to weigh up those elements.
"For me it's a clear choice, we voted to leave, we have to leave, some voters are frustrated we haven't left already."
Meanwhile, Garda Commissioner Drew Harris has said "zero work" has been done around the implementation of a hard border.
Mr Harris was speaking at the quarterly meeting of the Galway City Joint Policing Committee this afternoon.
He said a hard border was not on the agenda of An Garda Síochána.