Britain’s top judge has demanded clarity from ministers about how the UK judiciary should handle European court decisions after Brexit.
Supreme Court president Lord Neuberger said judges should not face the blame for misinterpretations “when parliament has failed to do so”.
The issue hinges on how judges are to interpret case law from the European Court of Justice (ECJ) once its jurisdiction over Britain ends.
It comes as a separate report says the Supreme Court will be able to block EU requests to hand over criminal suspects after Brexit.
Theresa May has promised that the ECJ will have no power over the UK once Britain quits the European Union.
But UK courts will still have to use European case law to inform their decisions long after Brexit.
Lord Neuberger, who steps down from his role next month, told the BBC that parliament must “spell out” how judges should approach the issue.
"If [the government] doesn't express clearly what the judges should do about decisions of the ECJ after Brexit, or indeed any other topic after Brexit, then the judges will simply have to do their best," he said.
"But to blame the judges for making the law when parliament has failed to do so would be unfair.”
He added: "If the UK parliament says we should take into account decisions of the ECJ then we will do so.
"If it says we shouldn't then we won't. Basically we will do what the statute says."
A government spokesperson said: "We have been clear that as we leave the EU, the direct jurisdiction of the European Court of Justice in the UK must come to an end.
"However, we want to provide maximum certainty so the Repeal Bill will ensure that for future cases, UK courts continue to interpret EU-derived law using the ECJ's case law, as it exists on the day we leave the EU."
Elsewhere, Brexit secretary David Davis has said British citizens facing extradition to EU states will be unable to appeal to the ECJ after Brexit, according to the Daily Telegraph.
The paper says the move is a bid to satisfy Brexit-backing Tory MPs who want the Supreme Court to be the highest judgement body on all issues.
But Guy Verhofstadt, the European Parliament's chief Brexit negotiator, told the Telegraph the ECJ should continue to oversee EU extradition requests to maintain the “legal order” of the bloc.