According to an internal document seen by Politico, European laws should still apply to Britain despite the vote to ditch the EU.
The document reportedly calls for a transitional period between the UK and Brussels after Britain has formally left the bloc – during which the EU’s Charter of Fundamental Rights would theoretically still be applied in the UK.
The ECJ could be set to meddle in Theresa May's Brexit plans
The Charter covers the EU’s basic freedoms, including the right to freedom of speech.
However, it also includes provisions that were key in determining Britain’s vote to leave the EU, including free movement rights and the right of every citizen “to move and reside freely within the territory of the member states”.
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The news comes despite Theresa May’s call to take Britain out of the jurisdiction of the ECJ.
When she outlined her Brexit plans in January, the Prime Minister said Britain “will not have truly left the European Union if we are not in control of our own laws”.
She added: “Leaving the European Union will mean that our laws will be made in Westminster, Edinburgh, Cardiff and Belfast.
The ECJ headquarters in Luxembourg
“And those laws will be interpreted by judges not in Luxembourg but in courts across this country.”
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With Mrs May set to trigger Article 50 next week and begin Brexit talks, the influence of the ECJ is likely to be a key issue officials must resolve before Britain formally leaves the bloc.
Last month, the ECJ president said the body is likely to play a role in Brexit negotiations.
Koen Lenaerts said: “It probably will, one day or another, end up on the docket of the Court – not because of the Court, but because of parties bringing the case.”
He went on to suggest the ECJ could even amend any trade deals between Britain and the EU after the UK leaves the grasps of the Brussels bloc.