Law firm says Brexit can't happen without Parliament vote

Mishcon de Reya, one of London's biggest law firms, said it's representing a group of unidentified clients threatening to take legal action against the British government if it tries to initiate the process of leaving the European Union without consulting Parliament.

The formal start to Brexit talks is likely to be the triggering of Article 50 of the Lisbon Treaty.

Mishcon de Reya said in a statement that this process can only be started with Parliament's consent.

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The U.K. government's position is that it's a decision for whoever is prime minister after David Cameron tendered his resignation.

"The result of the referendum is not in doubt, but we need a process that follows U.K. law to enact it," Kasra Nouroozi, a partner at the firm, said in an emailed statement.

"Everyone in Britain needs the government to apply the correct constitutional process and allow parliament to fulfill its democratic duty, which is to take into account the results of the referendum along with other factors and make the ultimate decision."

The firm, which said it is acting on behalf of a currently anonymous group of concerned clients, has retained David Pannick, Tom Hickman, Rhodri Thompson and Anneli Howard to act in the matter.

The law firm said it has been in touch with government lawyers since June 27 to "seek assurances that the government will uphold the U.K. constitution and protect the sovereignty of Parliament in invoking Article 50."