With Mrs May struggling to cobble together a working majority following a disastrous election, leading figures across the EU turned the screw on the Prime Minister.
European Council President Donald Tusk warned Mrs May there was “no time to lose” in the Brexit negotiations.
With talks due to start in Brussels in ten days time, Mr Tusk said it was their “urgent task” to get on with the negotiations in “the best possible spirit”.
In a letter to the Prime Minister congratulating her on her reappointment, he said the two-year time frame set out under Article 50 of the EU treaties left no room for delay. Talks are scheduled to begin a week on Monday.
“Our shared responsibility and urgent task now is to conduct the negotiations on the UK’s withdrawal from the European Union in the best possible spirit, securing the least disruptive outcome for our citizens, businesses and countries after March 2019,” he said.
“The time-frame set by Article 50 of the Treaty leaves us with no time to lose. I am fully committed to maintaining regular and close contact at our level to facilitate the work of our negotiators.”
Further pressure came from the German EU Commissioner Guenther Oettinger, who said the deadline for completing negotiations should be tightened.
Mr Oettinger said talks on the UK’s exit from the European Union must actually be finished by October next year, instead of March 2019, to allow the 27 remaining nations to approve the deal.
Oettinger said that “time for Brexit negotiations is getting tight” and added that “they must be closed in October 2018” to allow for the complicated approval process in the member states to run its course ahead of the official two-year deadline.
The two years was already seen as a tight proposition; slashing another six months off would make it even tougher.
European Commission president Jean-Claude Juncker said he hoped there would be no further delay to the negotiations.
“As far as the commission is concerned, we can open negotiations tomorrow morning at half-past nine,” he said.
“I do hope that the result of the election will have no major impact on the negotiations we are desperately waiting for.”
Earlier, Brussels’ chief negotiator Michel Barnier has said the talks would begin when Britain was ready, suggesting he would consider a short delay.
In a message released on Twitter, Mr Barnier said: “Brexit negotiations should start when UK is ready; timetable and EU positions are clear. Let’s put our minds together on striking a deal.”
There was clear frustration with the EU at the failure of the election to deliver a decisive result.
The European Parliament’s Brexit co-ordinator, Guy Verhofstadt, said: “Yet another own goal, after Cameron now May, will make already complex negotiations even more complicated.”
Germany’s European Commissioner Gunther Oettinger said “a weaker partner weakens the whole thing”, while if both sides were strong “you get results more quickly”.
German foreign minister Sigmar Gabriel said he hoped the election result would be seen as a message that the British people do not want a hard Brexit.
He urged the Government to “reconsider whether it’s really good for Great Britain to withdraw from the European Union in this way”.
He said he hoped the new Government was one “with which we can conduct serious negotiations and if possible keep Great Britain as close as possible to the European Union”.