Britain is awaiting an imminent ruling in the European Union Court of Justice on its challenge to powers handed to a finance watchdog to ban short-selling, in the first conclusion to several banking-centred legal battles between Westminster and Brussels.
The European Securities and Markets Authority (ESMA) has emergency powers to force a ban on short-selling in individual EU member states if it deems there is a threat to financial stability across the continent.
Though the UK backs the rules, it is pressing for clarity on how they fit with principles established under EU law.
It wants to be sure that states facing a short-selling ban cannot trump the regulator with a legal challenge and prevent immediate action to stabilise markets in the face of a potential crisis. ECJ judges' ruling on ESMA's short-selling powers will come on 22 January.
In September a senior legal adviser to the ECJ, Advocate General Niilo Jaaskinen, backed what the UK had flagged as a concern in the legitimacy of ESMA's short-selling powers.
Jaaskinen said they "go beyond what could be legitimately adopted as a harmonizing measure necessary for the establishment or functioning of the internal market."
Short-selling is the trading strategy of borrowing a certain amount of stock at one price and selling it on, hoping the value falls, then replacing the stock with the borrower at a lower price. The trader's profit is in the difference between the borrowed price and that at which they replace the stock.
The UK's legal challenge is one of several against the EU's attempts to regulate the financial sector. Most notably it is also battling attempts to bring in an EU-wide financial transaction tax and a cap on bankers' bonuses.
Eleven of the EU's 28 member states, including big-hitters Germany and France, want to tax certain financial transactions (FTT) made every day across Europe.
Under the FTT proposal, shares and bonds transactions would be taxed at 0.1% and derivatives at 0.01%. The EU estimates this would generate revenues of €30-35bn a year.
However the UK has mounted a legal challenge claiming the proposed FTT rules infringes national sovereignty on tax law. Britain is keen to protect London's status as the financial capital of the world and sees the FTT as a threat to its dominance.
"We're not against financial transaction taxes in principle but we are concerned about the extra-territorial aspects of the Commission's proposal and I think that concern is shared by some other countries," said UK Chancellor George Osborne at the time he launched the legal action in April 2013.
Separately the EU wants to cap bankers' bonuses at a maximum of double their salary by 2015, something the UK has also challenged in the European courts.
The UK argues this goes beyond the authority allowed under existing EU treaties. Britain wants to retain power over bonuses and is broadly against caps because shifting financiers' pay to fixed salaries makes it near-impossible to claw back remuneration in the event of misconduct.
Various legal disputes between the UK and EU come with the backdrop of a referendum over Britain's membership of the single market.
If the Conservatives win the 2015 general election they will hold an in/out referendum.
It is their bid to stave off political competition from the anti-EU UK Independence Party (Ukip), which is hoovering up votes from the Tories as it grows in popularity.
There is widespread concern in the UK that Brussels holds too much power, is undemocratic and a bureaucratic mess.
Proponents of the UK staying in the EU point to the economic benefits of belonging to the single market, such as the free movement of labour and free trade with the continent, the country's biggest export market.
Prime Minister David Cameron, a Conservative, said he wants to stay in the EU as long as it reforms itself and hands back powers to individual member states.