She will say: The economic and constitutional dislocation of a formal "third country" customs border within our own country is something I will never accept and I believe no British prime minister could ever accept.
Last winter the United Kingdom did agree to a backstop which promised regulatory alignment across the island in the event of a no deal Brexit - a move which allowed Brexit talks to progress at a key stage.
That move could complicate Ireland's demand for a backstop agreement that could have kept Northern Ireland but not the rest of the United Kingdom in an EU customs union.
One minute we're not getting a hard border, and the next minute we are.
His comments follow a number of amendments to the UK's Brexit plans which saw the insertion of a legal guarantee that there will be no post-Brexit customs border between Northern Ireland and Great Britain.
She will re-state that the proposal would breach the Belfast Agreement.
"All we are seeing is the use of legislative process in Westminster to make political points from different factions within the Conservative Party and within the British political system and I think it would be very foolish to overreact to that".
She also says it is clear that the British House of Commons would reject it.
"While she doesn't agree with the EU's wording in terms of the proposed Irish backstop to date, that doesn't mean that Britain isn't committed to a backstop".
In a special Brexit meeting at Derrynane House in Co Kerry, Mr Varadkar and his ministers thrashed out Brexit plans at the ancestral home of Irish political leader Daniel O'Connell.
Politicians have warned that the re-imposition of physical infrastructure on the border when it becomes the EU's external frontier would anger Irish nationalists in Northern Ireland who aspire to unification with the Republic of Ireland and help militants opposed to the peace deal to recruit new members.
"Clear UK commitments were made on this in Dec+March".
If UK Govt don't support current European Union wording on Backstop in draft Withdrawal Agreement, then obligation is on them to propose a viable and legally operable alternative wording that delivers same result: no border infrastructure.
British Prime Minister Theresa May is on her first visit to the province since the referendum vote.She's been criticised for not meeting with locals living and working near what will become the UK's only land border with the European Union after Brexit.The border is a crucial hurdle in the government's negotiations with Brussels.
The UK Government insists that any backstop position should include the UK as a whole.
On Thursday, the UK's new Brexit secretary Dominic Raab had his first meeting with the EU's chief negotiator in Brussels.