Theresa May has said any decision on a hard Irish border could be largely out of Britain's control.
The British Prime Minister has been taking questions at a House of Commons committee on Brexit.
The deal for the UK to leave the EU is being scrutinised by senior British politicians.
Asked about any scenario that could see a hard border, Mrs May said it is mostly out of their control.
She said: "We are not the only party to this arrangement. Obviously, there is the Irish Government, in fact, competence in this is a matter for the European Union.
"We would do everything we could not to erect a hard border. It would be a decision for the European Union and the Irish Government, and the concern that they would have would be about the fact that we would then be in a different set of circumstances on customs."
Appearing before the Commons Liaison Committee, Mrs May urged MPs to focus on her Brexit deal ahead of next month's crucial House of Commons vote.
The British Prime Minister said the only deal on offer is the one she has negotiated with the EU.
"My focus is on the vote that will take place on December 11 here in this House," she said.
"You want to look at all sorts of options and ideas. I think it is important Members of Parliament focus on the nature of this vote.
"This is an important point in our history. It is a vote on which we will be deciding whether we deliver on the decision of the British people.
She has rejected the suggestion passing her Brexit plan in Parliament without the support of the DUP will lead to the end of her partnership with the Northern Irish party.
Its leader Arlene Foster reiterated this morning that the party's 10 MPs would not back the PM's Withdrawal Agreement, saying it would create a "huge democratic deficit" in the North.
Speaking to the Commons Liaison Committee, the PM said: "Actually, the DUP have themselves said that the confidence-and-supply agreement remains in place."
However, Ms Foster has called on Mrs May to "not waste time" on her Brexit deal and instead look for a better solution to the Irish border.
Speaking to BBC Radio 4's Today programme, Ms Foster said the current proposals contain "68 pages of regulations that will apply to Northern Ireland and will not apply to the rest of the UK".
"There is a huge democratic deficit coming our way if we agree to this deal, because we've no say over the rules that will apply to Northern Ireland," she added.
Ms Foster said Theresa May's Brexit deal would see Northern Ireland "diverge off from the United Kingdom" as she called on her to "look for a better deal" that "takes back control of our money, of our borders and of our laws".
The DUP leader said: "All the things that made us vote for Brexit are the things that are going to be imposed on Northern Ireland."
She also hinted that if Mrs May's current Withdrawal Agreement fails to get through the Commons, her party could be open to a so-called "Norway-plus" style arrangement, where the whole of the UK stays in the customs union.
Ms Foster says the DUP's "one red line" is to make sure Northern Ireland is not differentiated from the rest of the UK in terms of customs and is "not prescriptive" about other potential options on the future relationship with the EU.
- Digital Desk and PA