Law firm Mishcon de Reya said it had started legal action to demand the British government win legislative approval from Parliament before triggering a formal divorce from the European Union.
Mishcon de Reya, a London-based law firm, said Britain would need an Act of Parliament - which would require approval votes by lawmakers in both houses of Parliament - before it invoked Article 50 of the Lisbon Treaty, the trigger for formal negotiations on exiting the bloc Britain joined in 1973.
The law firm said in a statement on Sunday it was acting on behalf of a group of clients but did not identify them. The BBC said the clients were a group of business people and academics.
Mishcon de Reya disputed the government's suggestion that following the 52-48 British referendum in favor of leaving the EU on June 23, it has sufficient legal authority to trigger Article 50 without recourse to parliament.
"The outcome of the Referendum itself is not legally binding and for the current or future Prime Minister to invoke Article 50 without the approval of Parliament is unlawful," Kasra Nouroozi, a partner Mishcon de Reya, said in the statement.
It has been in correspondence with government lawyers since June 27 to seek assurances for its clients that the government will respect the sovereignty of British parliament.
Conservative Prime Minister David Cameron has said he will resign and that it will be up to his successor to decide when to trigger Article 50.
Theresa May, who is currently ahead in the Conservative Party's leadership race, said that the will of the people had been clearly expressed and that "Brexit means Brexit".
(Reporting by Sarah Young; Editing by Guy Faulconbridge)