All MPs must swear allegiance to the Queen
Anti-monarchy campaigners hope to force a legal challenge to the oath of loyalty MPs swear to the Queen.
Human rights lawyer Louise Christian is representing campaign group Republic in its planned legal challenge.
Meanwhile 22 MPs have signed a Commons motion by Lib Dem Norman Baker, backing an alternative oath in which MPs would swear allegiance to their constituents.
Tory MP Douglas Carswell told the BBC it appeared human rights lawyers were trying to "usurp the Queen".
After election, MPs must swear an oath of allegiance "to Her Majesty Queen Elizabeth, her heirs and successors" or take a "solemn affirmation", in which they pledge to do the same, but without the words "I swear by Almighty God".
But there have been several protests against the oath - the Labour MP and former sports minister Tony Banks crossed his fingers as he read out his pledge and Sinn Fein members' refusal to take the oath means they are unable to take their seats in Parliament.
Republic seeks to challenge the oath sworn by MPs, as well as other officials like judges, people seeking citizenship, peers, archbishops and bishops and police constables.
They hope to force a judicial review through a test case where a new MP, magistrate or other official refuses to take the oath.
Ms Christian told BBC Radio 4's Today programme she believed the oath could be challenged on the grounds that it no longer reflected the way the country was run and that it discriminated against Catholics, Muslims, other religions and atheists by requiring them to swear allegiance to the head of the Church of England.
She said: "You can be perfectly patriotic without wanting to swear allegiance to... not just one person, it's not just the Queen it's her heirs and successors, whoever they may be."
But Mr Carswell told the BBC the case illustrated why Britain should adopt its own "bill of rights" and drop the Human Rights Act.
"The Queen is a profoundly unifying force, no matter what your background, what your heritage, where your grandparents came from, we can all unite with her as our personal head of state," he said.
He added: "Not content with trying to run public policy and things that should be left to the elected government, here we have an example of human rights lawyers wanting to usurp the Queen."
He argued that taking away the things that make the country distinctive would make the country indistinct.
But Ms Christian added: "What I was talking about was the rule of law and the importance of the rule of law and that means nobody is above the law."
The early day motion proposed by Mr Baker says that "some honourable members would prefer to swear an oath of allegiance to their constituents and the nation rather than the monarch" and calls for an optional alternative to be brought forward.
It has so far been signed by 22 MPs - 14 of them Labour, seven Lib Dem and one Conservative.