A European flag is flown as thousands of protesters gather in Parliament Square in London on July 2, 2016 | Niklas Hallen/AFP via Getty Images No Brexit without UK parliament vote: law firm Mishcon de Reya launches legal action to force Brexit to a parliament vote.

A British law firm is taking legal action to ensure the U.K. doesn't leave the EU without a debate and a vote in the House of Commons.

Mishcon de Reya, acting on behalf of an unspecified "group of clients," said Britain's EU referendum is not legally binding under U.K. law.

The law firm could be hoping MPs vote against the move to leave the bloc, as most back Remain.

While the British government "has suggested that it has sufficient legal authority" to start the process to exit the European Union, "the decision to trigger Article 50 of the Treaty of European Union ... rests with the representatives of the people under the U.K. constitution," Mishcon de Reya said in a statement. A decision by the government alone to withdraw from the EU would be "unlawful," according to the law firm.

Mishcon de Reya has been in contact with government lawyers since June 27, four days after Britain's vote on EU membership.

"We must ensure that the government follows the correct process to have legal certainty and protect the U.K. constitution and the sovereignty of parliament in these unprecedented circumstances. The result of the referendum is not in doubt, but we need a process that follows U.K. law to enact it," said Kasra Nouroozi, a partner at Mishcon de Reya.