Ministers are braced for a ruling by the European Court of Justice (ECJ) which could put further pressure on the Government to allow prisoners the right to vote.
The Luxembourg-based ECJ is widely predicted to deliver a ruling which, it is reported, could make it illegal for EU member states to impose a blanket ban on prisoners voting.
Such an announcement - on the eve of David Cameron's speech to the Conservative Party conference in Manchester - would be further setback for the Prime Minister as he seeks support for his plan to renegotiate the terms of Britain's EU membership.
Mr Cameron has previously said the idea that prisoners should be entitled to vote made him "physically sick".
The pressure on the Government could be further intensified by a preliminary ruling by the ECJ in another case, brought by the European Commission, challenging the UK's "right to reside test" which EU migrants must pass if they are to claim some benefits.
Mr Cameron has made curbing the access of migrants to benefits a key plank of his renegotiation strategy.
The ECJ ruling on prisoner voting rights comes after the Government has ignored a series of rulings by the European Court of Human Rights - an entirely separate body - that Britain's blanket ban on prisoner voting is illegal.
The Government's hand was strengthened by a Commons vote in 2011 when MPs overwhelmingly supported the retention of the ban.
The latest case has been brought by a former French prisoner, Thierry Delvigne, who is claiming that a ban on him voting violated his right enshrined in the EU's Charter of Fundamental Rights entitling citizens to take part in European elections wherever they live in the EU.
A Downing Street spokesman said: "Whatever decision is reached, we will study that judgment."