Boris Johnson insisted a hard Irish border would be "madness" as Labour urged him to set a red line or prepare to resign over the issue.
The foreign secretary told the Commons it would be "unthinkable" for a return to a hard border between Northern Ireland and the Republic, adding all MPs understand the "social, political and spiritual ramifications" of such a move.
He also said it is important for the second phase of Brexit talks to begin, with a need for "sufficient progress" at next month's European Council.
But shadow foreign secretary Emily Thornberry said Mr Johnson has, as with other promises over Brexit, gone from giving an "unequivocal guarantee to an aspiration dependent on a successful deal".
The Labour frontbencher said it appeared Mr Johnson was suggesting it is for the Irish to find a solution.
She challenged Mr Johnson to announce a red line and promise unequivocally that Irish border arrangements will not change or say he will refuse to stay in the government if they do.
Speaking in the Commons, Ms Thornberry reminded Mr Johnson of his February 2016 pledge that Brexit would leave Irish border arrangements "absolutely unchanged" and asked him to give the same promise.
Mr Johnson, in his reply, said: "There's no question the issue of the border is very live in Irish politics.
"I repeated exactly the pledge to which she refers, that there can be no return to a hard border, there can be no hard border.
"That would be unthinkable.
"It would be economic and political madness.
"We certainly, I think everybody on both sides of this House, understand the social, political and spiritual ramifications of allowing any such thing to happen.
"That's why it's so important that we get on to the second phase of the negotiations, that we get sufficient progress at the European Council in December and we're able to debate these issues properly."
Ms Thornberry said Mr Johnson had shifted from his original promise, noting: "It seemed to me like his jogging partner from The Sun (editor Tony Gallagher) he now seems to be saying that it's up to the Irish to find a solution, but why should that be?"
Ms Thornberry added: "In September he laid down four personal red lines for the Brexit negotiations and none of them related to the Irish land border, which is a crucial issue to 1.8 million of our own citizens and 4.8 million of our friends south of the border.
"Let me urge him today to announce a fifth red line and to promise unequivocally what he promised last year, that Irish border arrangements won't change, and if they do he will refuse to stay in the government."
Mr Johnson said his Labour counterpart had prepared her follow-up question before his initial answer, adding: "We don't want a hard border north-south or indeed east-west."
