"In any political negotiations, there is not enough time, not enough money, not enough this, not enough that", Danish Finance Minister Kristian Jensen told the Guardian.
She also quizzed Mrs May on what discussions her government had held with the Spanish Government about the future of Gibraltar's border with Spain. "But I am optimistic we will receive a positive response", May said, in response to an EU Commission spokesman's earlier remarks that the ball was in the U.K.'s court.
At Westminster on Monday, the PM said that as hopes of a potential agreement seemed to be fading fast, at home and overseas, that it was time to detail out the government's "steps to minimise disruption" before March 2019.
On Thursday, Germany's BDI industry federation said companies must prepare for a "very hard" Brexit after a tumultuous UK Conservative Party conference that left Prime Minister Theresa May's authority weakened.
Even before this latest intervention, the prognosis for the talks was grim, with commission chief Jean-Claude Juncker having warned that "miracles" would be needed this week to make enough progress to get a positive decision at the summit.
As the fifth round of formal Brexit talks got under way in Brussels, Margaritis Schinas told reporters that "the ball is entirely in the United Kingdom court" to reach agreement on Britain's "divorce deal", without which the European Union has said it will not move on to the second phase of talks, involving trade.
Brexit is the term used for the UK's exit from the EU.
May, however, struck a confident tone during her speech on Monday, telling MPs: "I believe we can prove the doomsayers wrong".
May, meanwhile, is expected to say in a speech later on Monday to MPs in the Westminster parliament that "the ball is in their [EU] court".
Her survival has so far been dependent on the absence of an obvious successor who could unite the split party around Brexit and fear of an election that many Conservatives think would let opposition Labour leader Jeremy Corbyn into power. We do want to ensure that the trading relationship we have can be as tariff free and frictionless as possible.
- Britain will not be a member of EU institutions during the two-year "implementation" period after it leaves the union on March 29, 2019, but it will retain access to the EU single market until the implementation period is over.
This week's talks came as ministers sought to ease disquiet among Brexit-backing MPs about the UK's strategy for a two year "transition" period between being a full European Union member and the UK's eventual post-Brexit relations with the EU.
"It's important to continue that momentum and for the negotiations to move on to the next phase in order that we can discuss our future partnership".