A group of British expatriates is mounting a legal challenge to European Commission president Jean-Claude Juncker's refusal to discuss Brexit until the UK formally notifies Brussels of its intention to leave.

Mr Juncker used a presidential order to ban Brussels bureaucrats from holding talks with British representatives, insisting: "No notification, no negotiation."

READ MORE: First Minister Nicola Sturgeon urged to set out Brexit terms that would see her call off second Scottish independence referendum

Now the group Fair Deal for Expats have instructed Croft solicitors to mount a challenge to the ban at the General Court of the European Union in Luxembourg.

They denounced Mr Juncker's order as "dictatorial-style bullying" and are seeking funds through website Crowdjustice to pay for the legal action.

Article 50 of the Treaty on European Union states that a two-year process of negotiations on a member-state's withdrawal should begin after the departing country informs the European Council of its intention to quit.

Since the June 23 referendum vote for Britain to leave, Mr Juncker has repeatedly called for a swift notification to bring an end to uncertainty, but Prime Minister Theresa May has insisted she will not invoke Article 50 before the end of 2016.

READ MORE: First Minister Nicola Sturgeon urged to set out Brexit terms that would see her call off second Scottish independence referendum

In talks with the PM at 10 Downing Street on Thursday, Council president Donald Tusk told her: "The ball is in your court."

In a speech to the European Parliament shortly after the referendum, Mr Juncker said: "I have forbidden Commissioners from holding discussions with representatives from the British Government - by presidential order, which is not my style.

"I have told all the directors-general that there cannot be any prior discussions with British representatives. No notification, no negotiation."

Fair Deal for Expats spokesman John Shaw said the group wanted the UK to be free to negotiate with EU institutions before the invocation of Article 50, permitting an outline deal on withdrawal to be agreed as soon as possible.

"People in Britain and across Europe have become accustomed to Mr Juncker's antagonistic style and bombastic rhetoric," said Mr Shaw. "But his dictatorial-style bullying tactics will not be tolerated. Enough is enough.

READ MORE: First Minister Nicola Sturgeon urged to set out Brexit terms that would see her call off second Scottish independence referendum

"Mr Juncker is urging Britain to trigger Article 50 sooner rather than later, but he's not entitled to issue edicts preventing the UK from having discussions with the Commission. He's forgetting that the UK is still a member state."