The Liberal Democrats have warned the UK could face difficulty securing a post-Brexit trade deal with the EU after IBTimes UK shed light on a little-known court case that could have major ramifications on the issue.

"It's clear that negotiating a trade deal with the EU post-Brexit will be nowhere as easy or simple as the Leave campaigners claimed," said Tom Brake MP, the party's foreign affairs spokesman.

"The reality is any one country may well be able to veto the deal. The best way to secure the UK's economic interests and protect jobs and investment is to stay in the single market."

The comments come after details of a case at the Court of Justice of the European Union (CJEU), known as 'Opinion 2/15', were revealed. The hearing is in English, but unusually references to it on the CJEU website are only in French.

EU law expert David Kleimann, who sat through the case for two days in September, claimed no one was allowed to take any pictures or recordings during the hearing. "I'm amazed, because it's such an important case," he told IBTimes UK.

On top of that, all 28 of the CJEU's judges are presiding over the case. Normally three to 15 judges oversee a hearing.

The case concerns a free-trade agreement (FTA) between the EU and Singapore. The point of contention is whether the European Commission, the executive body of the EU, could negotiate and conclude the deal and avoid asking all 28 nations of the economic bloc.

"[It] could potentially decide whether EU Member States have a veto in negotiating the new trade relationship with the UK," said Dr Andrés Delgado Casteleiro, a researcher at Luxembourg's Max Planck Institute.

The revelation came as British Prime Minister Theresa May is expected to meet European Commission President Jean Cluade Juncker at the EU Council summit in Paris today and tomorrow (20-21 October).

May has ruled out giving a "running commentary" of her government's negotiating strategy with the EU. But the Conservative premier has promised to trigger Article 50, the official mechanism to split from the EU, by March 2017.

Fears have grown in the UK and Brussels over a so called "hard Brexit", which could see the UK splitting from the EU and resorting to World Trade Organisation (WTO) rules to trade with the bloc.

EU chiefs, including Juncker and European Council President Donald Tusk, has continually ruled out giving the UK full access to the single-market and curbs on free movement of people.

The Department for International Trade and the Labour Party had not responded to a request for comment at the time of publication.