The EU's chief Brexit negotiator apparently wants to hold talks with UK officials in French instead of English, in a move that is likely to anger British Eurosceptics.
French politician Michel Barnier, a former vice president of the European Commission, is reportedly hoping to use his mother tongue in meetings and documents.
"Barnier wants French to be the working language in Brexit negotiations with Britain," a source told Reuters.
But the EU has been quick to down play the report, with a commission spokeswoman confirming to IBTimes UK that there is no "language regime" for the negotiations.
"We will not comment until we receive a formal request," a Number 10 spokesperson told Reuters.
English remains the main working language of the EU, while French and German are also used by officials.
The EU was forced to confirm in June that English will continue to be an official language after the UK breaks away.
"Any change to the EU Institutions' language regime is subject to a unanimous vote of the council," the commission's representation in Ireland said.
The report of Barnier's alleged language preference broke as Jean Claude Juncker met Theresa May in Brussels.
The commission president and UK prime minister held the talks after the end of the latest EU council.
The Brexit issue was not on the main agenda, but May addressed her European counterparts at a working dinner last night.
The conservative premier stressed Britain should play a full role within the EU's decision-making. The comments come after the EU's informal Bratislava summit in September, which saw the exclusion of the UK.
May has promised to "work closely" with the EU after the UK breaks from Brussels. The prime minister recently revealed she would trigger Article 50, the official mechanism to split from the EU, by March 2017.
But her government is currently facing a challenge in England's High Court over whether ministers have the legal authority to make the move without holding a parliamentary vote.
UPDATE: 14:43 BST
Barnier responds to report on Twitter.