Allies of Mrs May believe Germany, in tandem with the EU, is embarking on a new 'project fear' by repeatedly briefing against her.
Senior officials in the German government and in Brussels have openly mocked Mrs May or leaked sensitive information about private meetings in what is being seen as an attempt to undermine the Prime Minister.
Last week Angela Merkel, the German Chancellor, also suggested Britain had “illusions” over what it could hope to achieve from Brexit.
Weakening Mrs May’s mandate with the electorate would tilt the balance in favour of the EU negotiators, and Conservative sources have suggested she is the victim of a co-ordinated plot ahead of next month’s election.
One close ally of Mrs May said: “There is a long-standing tradition that countries do not involve themselves in the elections of other countries, and they seem to be breaking that.”
Asked by the BBC if she was taking a “realistic” approach over the negotiations, Mrs May said: “I think what we've seen recently is that at times these negotiations are going to be tough.
“Now 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.”
Martin Selmayr, the German chief of staff to Jean-Claude Juncker and a personal friend of Angela Merkel’s right-hand man, is believed to be behind a leak that claimed Mrs May was “deluded” over her Brexit demands.
Michael Roth, Germany’s finance minister, also sought to undermine Mrs May, using Twitter to say: “The British government must abandon [the] myth that all British will be better off post-Brexit.”
And Guy Verhofstadt, the lead Brexit negotiator for the European Parliament, has openly mocked Mrs May’s negotiating skills.
Picking up on her campaign slogan of “strong and stable leadership”, he said: “Any Brexit deal requires a strong & stable understanding of the complex issues involved. The clock is ticking - it's time to get real.”
Tory MP Sir Bill Cash, chairman of the Commons European scrutiny committee, said he was “certain” that Germany and the EU were trying to influence the general election by undermining Mrs May.
He said: “What they are doing is trying to exploit a new kind of 'project fear' and that is not going to work on the British people.
“They are also trying to use negotiations as a means of influencing the German general election later this year. They are playing an unwise and dangerous game and I think they have been working towards this for a long time.”
Margaritis Schinas, European Commission spokesman, said he would not comment on leaks, but added: "I have the impression sometimes that our British friends - not all of them - do underestimate the technical difficulties we have to face... there are issues that clearly are not understood the same way.”
Amber Rudd, the Home Secretary, insisted that the Government would not enter into a “briefing war” with Brussels because it would be a “mistake” to be drawn into one.
She said: "I'm not surprised that there is briefing coming out from different sides of a negotiation.
"But what we will always do is make sure that we conduct our negotiations more discreetly, shall we say, so that really we can have a freer negotiating hand.
"I think it's a mistake to allow those sort of details, if they are true, to come out from a dinner."
Yanis Varoufakis, the former Greek finance minister and an outspoken critic of the EU, told BBC Radio 4's World At One programme: "Nothing was more predictable than this kind of clash, the substantial policy and tactic of the Brussels politicians and bureaucrats of effectively stalling and constantly shifting the goalposts in any such negotiations."
He said any attempt to negotiate a mutually beneficial deal "is going to be defeated by the reality in Brussels".
Sir Keir Starmer, Labour's shadow Brexit minister, said: "I think the talks have got off to a very, very bad start.
"That rigid approach, that fixed approach that she has taken here in the UK is simply not working with our EU partners.
"We need to be flexible, we need to be smart and we need to understand how negotiations go.”