After June's referendum, the bureaucrat blocked his Commissioners from holding talks with British officials until Article 50 is triggered.
Meddling Mr Juncker claimed he was using a "Presidential Order" to implement the ban – even though no such order exists.
Now a campaign group is set to take the Commission to court, claiming the move "discriminates against the UK and its people".
Lawyers representing the Fair Deal for Expats group will also argue it "infringes the rights of EU citizens who live in another EU country".
The case is expected to be heard by the EU's General Court in Luxembourg, according to the Sun.
If the legal action is successful, the Government would be able to begin Brexit talks with Brussels without having to invoke Article 50.
The result would be humiliating for the unelected fatcat, who is blamed by some EU officials for Britain's historic vote to quit the bloc.
He issued several doom-mongering warnings about Brexit earlier this year, saying Britain would be treated as a "third world country" outside the EU.
And last week he sensationally attacked Theresa May for holding preliminary trade talks with Australia at the G20 summit.
Speaking at summit in China, the Mr Juncker said: "I don't like the idea that member states of the EU are negotiating free trade agreements."
Last night a Commission spokesman insisted it was "clear" talks between the EU and Britain cannot start until Article 50 is invoked.
He added: "President Juncker has instructed Commissioners and EC officials also to follow that principle."
