The President of the European Commission apparently advised the British Prime Minister that her 17-seat majority would not be enough during exit talks.
Mr Juncker allegedly claimed a larger majority would help Ms May during “pinch points”, such as establishing the sum of the UK’s divorce bill.
But it seems Mr Juncker’s advice has now served to put Mrs May on a weaker footing as negotiations approach, as she faces calls to resign.
A European Union source claimed: “During bilaterals, in the margins of summits, Juncker repeatedly told her he thought she should do it.”
A second diplomat told the Observer: “People don’t understand. We want a deal more than anyone. We are professionals, we have a mandate to get a deal and we want to be successful in that.”
Mr Juncker and Mrs May’s relationship has become increasingly strained in recent months, made even worse after it was revealed his team leaked details of a dinner between the pair at Downing Street.
The EU Commissioner reportedly told Mrs May as he left the dinner: “I leave Downing Street 10 times more skeptical than I was before.”
The British PM then claimed in a speech EU officials were deliberately attempting to swing the result of the election on June 8th by undertaking such leaks.
Theresa May called the election in April, hoping to secure a bigger majority than the one she inherited from former PM David Cameron.
But her decision backfired on a huge scale following a disastrous election campaign and a huge surge in support for Jeremy Corbyn.
Faced with a minority government, Mrs May is now being forced to secure a “confidence and supply” deal with hardline DUP in northern Ireland – a move proving hugely controversial among the electorate.
A petition has already secured more than 600,000 signatures calling for the supposed coalition to be cancelled.
The revalations come as EU Commissioner Phil Hogan said the results of Thursday’s election proves Britons are opposed to “the notion of a hard Brexit”.
He said leaving the customs union would be “economic suicide” and that the next government would likely opt for a soft Brexit instead.
The move would infuriate anti-EU campaigners, who want Britain to fully cut ties with Brussels and strike trade deals with the rest of the world.