Boris Johnson, the frontrunner to become Britain's next prime minister, has refused to rule out suspending parliament to deliver on his promise that Britain will leave the European Union on Oct. 31 with or without an exit deal.

Parliament has on several occasions voted against a no-deal Brexit, meaning that unless Johnson could secure a new exit agreement with the EU and get it approved by lawmakers, he may have to resort to suspending the legislature.

That prospect has angered fellow lawmakers and campaigners who say parliament should not be bypassed.

Miller, who is best known for successfully taking the government to the Supreme Court to challenge its authority to leave the EU without a vote in parliament, said she had reassembled the same legal team for a fresh challenge.

"We are ready to go to court to test the legality of any attempt to prorogue parliament," she said in a statement.

Former prime minister John Major also said last week he would be prepared to turn to the judiciary if needed to avoid a constitutional crisis.

Johnson is favourite to win the contest to take over from Prime Minister Theresa May.

The winner is announced on July 23.

His first preference is to renegotiate a new deal with the EU and have it approved by parliament, but he has said it would be unwise to rule out anything - including suspension - that might be needed to deliver Brexit.

In a letter to Johnson, Miller's lawyers at law firm Mishcon de Reya argued that in the current circumstances it would be "constitutionally objectionable" and unlawful to ask the queen to suspend parliament - a process called prorogation.

"It would seriously undermine parliamentary sovereignty for you, as prime minister, to prorogue parliament to prevent it from considering whether to legislate to prevent a no-deal Brexit," the letter said.

It called on Johnson to rule out suspending parliament and that if he would not, asked that he gave enough notice to allow any such move to be tested in the courts.