The EU Commission says a hard border can only be avoided if Northern Ireland stays in the single market after Brexit.
A position paper sent by the EU to the British government says the North should also stay in the customs union.
It says this is the only way that the common travel area can be maintained, and the Good Friday Agreement protected.
Sinn Féin says the paper is a sign the Commission supports the idea of securing special status for Northern Ireland within the EU.
It comes as the British government announced that the 'exit day' will be 11pm on March 29, 2019.
The date of Brexit will be written into law as Theresa May warned Tory rebels that the process of leaving the European Union will not be derailed.
And the Prime Minister issued a blunt message to pro-EU MPs, saying any attempt to block Brexit will not be tolerated.
The move comes as Mr Davis was set to meet the European Union's chief negotiator Michel Barnier in Brussels, where officials have been engaged in the latest round of Brexit talks.
The amendment enshrining Brexit day in law will be considered by MPs when the European Union (Withdrawal) Bill returns to the Commons next week.
Writing in the Daily Telegraph the Prime Minister said: "We will not tolerate attempts from any quarter to use the process of amendments to this Bill as a mechanism to try to block the democratic wishes of the British people by attempting to slow down or stop our departure from the European Union."