British expats living in the EU are suing European Commission President Jean-Claude Juncker over his order to prevent informal Brexit discussions.
Mr Juncker insists there can be no negotiations until the UK triggers Article 50 - the withdrawal process.
The expat group wants immediate talks about the implications of Brexit for Britons living in other EU countries.
UK Prime Minister Theresa May has pledged to trigger Article 50 by the end of March.
But she also called for "preparatory work" with EU governments - before the official negotiations - to help smooth the process of UK withdrawal.
The non-profit association Fair Deal for Expats has issued a legal challenge against Mr Juncker at the European Court of Justice (ECJ). The group alleges that his "presidential order" concerning Brexit discussions is an "unlawful gagging order" which "must be annulled immediately".
Croft Solicitors, representing the expats in the case, told the BBC that "there is no such thing as a presidential order" in the European Commission.
Mr Juncker and some other European politicians have warned against an "a la carte" deal for the UK which might encourage other EU members to cherry-pick EU policies, unravelling the rulebook.
'No prior discussions'
In a speech on 28 June, four days after the Leave campaign won the UK 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 [Commission] Directors-General that there cannot be any prior discussions with British representatives. No notification, no negotiation."
On Friday Mr Juncker insisted again that the EU must not compromise on free movement of labour during the Brexit divorce talks.
It is among the EU's cherished four freedoms - the others being free movement of capital, goods and services - that members of the EU single market have to respect.
Speaking in Paris, Mr Juncker said "you can't have one foot in and one foot out".
"On that point we must be intransigent. I can see the manoeuvring."
Many UK politicians want a deal that would allow the UK to keep key single market advantages while also limiting the numbers of EU migrants entering the UK.
Mr Juncker warned against "secret talks in dark rooms, with curtains drawn, with British government representatives".
"If we start unravelling the [EU] internal market... we will set in train the end of Europe," he said.
There are just under 1.2m UK citizens living in other EU countries, the largest group being in Spain (just under 310,000), followed by Ireland (255,000) and France (185,000), according to United Nations data.
Wynne Edwards, a member of Fair Deal for Expats living in France, said Mr Juncker's "order" was an attempt to influence legitimate discussion on Brexit among UK citizens, and so amounted to discrimination.
"If Germans wanted to speak about the effects of Brexit on Germany they wouldn't be prohibited from doing so," he told the BBC Radio 4 Today programme.
Healthcare is among the key concerns of British expats, he said, citing the case of a Briton who had been getting cancer treatment in France, but had been refused some medication in the UK.
"We ought to be allowed to find out what the lines in the sand are, what is negotiable and what isn't," he said.