Legal action is being launched to try and stop Britain beginning the process of leaving the EU, without a new act of Parliament.
Law firm Mischon de Reya says it is acting on behalf of a group of academics and business leaders.
It believes it would be unlawful for a UK Prime Minister to trigger the Article 50 process which leads to a Brexit, without putting it to a parliamentary vote.
This after Britain voted to leave the European Union in a referendum last month.
Political reporter Theo Usherwood says Mischon de Reya's case has a technical point.
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OUT: …the EU."
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Meanwhile, Boris Johnson says the government must start setting out the potential upsides of an EU divorce.
He's making the call after stepping out of the Conservative Party leadership race, despite having been a figurehead of the Vote Leave campaign.
Political reporter Theo Usherwood says it shows Boris is not retreating completely.
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OUT: …Boris Johnson."
DUR: 44 seconds