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.”