May made the comment yesterday after day-long debates over a reportedly "disastrous" dinner meeting between her and European Commission President Jean-Claude Junker at Downing Street last week.
"I think what we've seen recently is that at times these negotiations are going to be tough. During the Conservative Party leadership campaign, I was described by one of my colleagues as a bloody difficult woman. And I said at the time the next person to find that out will be Jean-Claude Juncker," 60-year-old May told the BBC.
Her fierce reaction was in reference to a German newspaper, 'Frankurter Allgemeine', report based on allegedly leaked information that May and Juncker had clashed at Downing Street over the British PM's desire to make Brexit "a success" and on whether the issue of protecting the rights of expat British and EU nationals could be agreed as early as June.
Juncker is reported to have told the British Prime Minister as he stormed out of the meeting: "I'm leaving Downing Street 10 times more sceptical than I was before."
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Meanwhile, the EU's chief Brexit negotiator, Michel Barnier, unveiled the EU's Brexit plans on Wednesday, warning that Britain must not hold any illusions of it being a quick and painless 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. This is not the case. We need sound solutions, we need legal precision and this will take time," he said in Brussels.
His announcement came as UK's minister for exiting the EU, David Davis, categorically denied media reports that Britain could end up paying as much as 100 billion euros as part of the so-called divorce settlement with the EU.
The minister said that various figures ranging from 50 billion to 100 billion euros had been knocking around but there was no official estimate so far.
"We will not be paying 100 billion euros. We will do it properly. We will take our responsibilities seriously. What we've got to do is discuss in detail what the rights and obligations are," he said.
Many Conservative MPs argue the UK does not owe the EU anything given the size of the contributions it has made over the past 40 years.
A recent report by a House of Lords committee argued the UK was not legally obliged to pay a penny although to do this would threaten any chance of a post-Brexit trade deal.
In 2016, the UK had voted to leave the EU. The UK government started the withdrawal process on March 29 2017, putting the country on course to exit by April 2019.