Speaking outside 10 Downing Street, after seeing the Queen to mark the dissolution of parliament ahead of the June 8 general election, May said the EU stance on Brexit had "hardened." She accused unnamed EU politicians and officials of making "threats against Britain."
The "misrepresentation" of Britain's Brexit negotiating position was timed to affect the election outcome, she said.
May's tough rhetoric came after damning accounts emerged of a Downing Street dinner last week with EU Commission President Jean-Claude Juncker. EU officials said the dinner went badly
, claiming British officials were in a "different galaxy" over issues such as the UK's financial obligations and the status of European nationals in Britain.
An EU source told CNN that May's government "simply does not seem to get it." If her negotiating position did not change after the June 8 election, then "Houston, we have a problem," the source said.
In her remarks on Wednesday, May said the negative briefing was deliberately aimed at influencing the outcome of the UK election.
"Britain's negotiating position has been misrepresented in the continental press. The European Commission's negotiating stance has hardened. Threats against Britain have been issued by European politicians and officials."
"All of these acts have been deliberately timed to affect the result of the general election that will take place on 8 June."
May said she had made clear in her letter triggering the Brexit process in March "that in leaving the European Union, Britain means no harm to our friends and allies on the Continent."
She added, "We continue to believe that no deal is better than a bad deal -- but we want a deal."
'Bloody difficult woman'
The dinner reports first emerged in Politico Europe on Friday and were turbocharged by a full-page story in the German newspaper Frankfurter Allgemeine on Sunday.
After initially dismissing the accounts as "Brussels gossip," the Prime Minister told the BBC on Tuesday, that if the EU tried to play hardball, they would find her a "bloody difficult woman."
On Wednesday, the Financial Times reported that Britain could face an upfront payment of up to €100 billion ($109 billion) to leave the EU.
Without confirming the figure, the EU's chief Brexit negotiator Michel Barnier warned that Britain's departure from the bloc would be a painful and costly process
"Some have created the illusion that Brexit would have no material impact on our lives or that negotiations can be concluded quickly and painlessly," Barnier said. "This is not the case."
The mood in Brussels towards the UK "has soured," an EU source told CNN.
"We're not feeling a lack of trust but a lack of confidence because [Theresa May] doesn't seem to understand the situation and sees the world through British glasses. I don't feel they see us. They are looking into a mirror and fail to perceive what is different."
The source expressed concern about being able to reach a deal in light of May's "extreme optimism," adding: "So far what they want doesn't translate into something we can conceive or understand. She has been saying we want our cake and eat it."