At a meeting on Monday, EU negotiators told European diplomats that changes suggested by the new UK government led by Boris Johnson, such as scrapping the Irish border backstop, were unacceptable, the BBC reported.
The backstop - agreed by May in November 2018 - would see Northern Ireland staying aligned to some rules of the EU single market.
They said the EU "needed to change its stance", as May's deal would not pass through Parliament in its current form.
"We will throw ourselves into the negotiations with the greatest energy and the spirit of friendship and we hope the EU will rethink its current refusal to make any changes to the withdrawal agreement."
The plan negotiated between the EU and the former Prime Minister was voted down by MPs three times.
New Prime Minister Johnson has pledged to "do or die", and leave the EU by the deadline of October 31, with or without a deal.
The meeting follows an interview where Health Secretary Matt Hancock had said that Parliament could no longer block the UK from leaving the EU without a deal, the BBC reported.
In a no-deal scenario, the UK would immediately leave the EU with no agreement about the "divorce" process, and would exit overnight from the single market and customs union.
Opponents have said that a no-deal exit would damage the economy and lead to border posts between Northern Ireland and the Republic.
Meanwhile, Foreign Secretary Dominic Raab will begin a tour of North America on Tuesday as part of a bid to "fire up" the UK's trade relationships with countries outside the EU.
He will visit Canada, the US and Mexico.
( With inputs from IANS )