The Irish premier added it was “alarming” that Conservative leadership candidates have proposed changes to the policy which safeguards against a hard border.
Boris Johnson, the frontrunner in the race to replace Theresa May as prime minister, is among those who have suggested ditching the backstop in favour of unspecified “alternative arrangements”.
But the EU has insisted the Withdrawal Agreement – including the backstop – is not up for renegotiation.
Mr Varadkar told RTE: "To me no backstop is effectively the same as no deal because what the backstop is is ... a legally operable guarantee that we will never see a hard border emerge again.
“If we don't have that, that is no deal."
The Democratic Unionist Party, which has supported Ms May's government in Westminster, has insisted it would not back any EU withdrawal deal which includes a backstop with no time limit.
But Mr Varadkar said: "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.
"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."
The Taoisearch added: "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 Johnson has claimed it is "perfectly realistic" to renegotiate the Withdrawal Agreement to allow the UK to leave the EU in October.
Speaking on BBC Radio 4's World At One programme, he described the backstop as the "fundamental flaw" in the agreement.
He added: "Those problems are easily capable of solution, as I think the [European] Commission has said in the past, with maximum facilitation techniques and, after all, at the moment you already have goods conforming to different standards."