Ireland's Taoiseach has said full regulatory alignment should be "spelled out" in this week's draft EU withdrawal agreement.
Leo Varadkar held a telephone conversation with the Prime Minister on Monday evening.
Under London's post-Brexit trade plan the UK would examine existing regulations and decide whether it wants to maintain, alter or abandon them in certain areas.
A spokesman for the Irish premier said: "The Taoiseach also repeated the necessity from the EU side to have the detail of the backstop option of full regulatory alignment spelled out in the draft legal text of the Withdrawal Agreement.
"This option would only come into effect if agreement on one of the other options is not reached."
They discussed Brexit and, in particular, the draft withdrawal agreement.
The spokesman added: "Both the Taoiseach and the Prime Minister said they want the options, as set out in the December Joint Report, to be examined in detail.
"This would include the preferred option of a satisfactory solution to the border problem being found within the overall future relationship between the EU and the UK."