The taoiseach (Irish prime minister) has said removing the backstop from the Withdrawal Agreement, would be "effectively the same as no deal".
Leo Varadkar was responding to comments from some candidates seeking to replace Theresa May as prime minister.
Many contenders have proposed changes to the backstop, even though the EU says it is not up for renegotiation.
Mr Varadkar said: "If we don't have that (the backstop), there is no deal".
It is an insurance agreement designed to avoid a hard border between the Irish Republic and Northern Ireland.
Many Conservative MPs have concerns that it could "trap" the UK, leaving it unable to strike its own trade deals with the rest of the world.
The Democratic Unionist Party (DUP), who prop up the government, also do not want to see Northern Ireland treated differently from the rest of the UK.
Speaking on Irish National Broadcaster RTÉ's Marian Finucane programme, the taoiseach said it was "alarming" some leading Conservatives were suggesting a no-deal Brexit.
"It's a legal guarantee and legally operable guarantee that we will never see a hard border again," Mr Varadkar said of the backstop.
He also responded to calls for a time limit to be attached to the backstop.
"The difficulties we have with a time limit, is effectively you are saying there will or could be a hard border once that time limit expires - that isn't a backstop," he said.
"What we are open to, and always have been open to, is alternative arrangements that perhaps could avoid a hard border, through procedures and technologies and so on.
"What we expect, and I don't think it's unreasonable - we want to see that fleshed out, we want to see it exist, it demonstrated before we are willing to give up the backstop.
"What people are saying is, 'give up the backstop' which we know will work legally and operationally in return for something that doesn't yet exist but might exist in the future.
"I can't do that to the border communities."
Mr Varadkar also said he was "concerned at the idea, and there is an idea there in Westminster, in London, that somehow Theresa May was a bad negotiator and got a bad deal.
"That's not true. She was a good negotiator, she had a good team.
"She probably got the best deal that she could get given that a country leaving the EU doesn't have much leverage.
"The fact that the failure of the House Of Commons to ratify the Withdrawal Agreement somehow means they are going to get a better deal, that is just not how the European Union works," he said.
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