MPs are debating changing the rules of royal succession, in a move towards gender equality which could also open up a can of legislative worms across the Commonwealth.
Critics say the seemingly uncontroversial bill would allow republican campaigners a chance to unsettle constitutional arrangements in Commonwealth countries and present new difficulties in the monarchy's fraught relationship with the Catholic Church.
"We should be looking carefully at the implications," Democratic Unionist party MP Ian Paisley said.
"Members assume they know what the intended consequences of the bill are. But there are unknown consequences.
"Recently in Northern Ireland we had a remove to remove a symbol of our state from a public building. People thought they knew what the consequences of that would be, but there have been 70 days and millions of pounds' worth of disruption."
Labour MP Paul Flynn, who added amendments to the bill calling for a vote on the future of the monarchy when the Queen dies, demanded more time for the session, saying "royalists, republicans and allegedly a member of the royal family" all wanted a more thorough debate.
Tory MP Jacob Rees-Mogg described the bill as the most important ever debated by the Commons and said rushing it through was "an insult to our nation, our sovereign and parliament".
Commentators marvelled at the bill's ability to unite figures as diverse as Ress-Mogg, Flynn and Paisley in opposition.
Paisley specifically cited the "unlikely coalition" between him and Rees-Mogg as one of a "high papist" and a "tight prod".
There was a brief moment of humour when home affairs chairman Keith Vaz assured the House he had "no plan to marry a royal heir", to which Tory Mp Nicholas Soames shouted: "He would if he could."
Nick Clegg, who led the bill through a lengthy consultation process with other Commonwealth countries, said: "The other Commonwealth countries where Her Majesty the Queen is head of state have just given us the green light to change the law, and we are wasting no time," Nick Clegg said last night.
The bill would allow an elder daughter to take the throne before a younger male sibling, but it also carries provisions on religion.
"The current law says our monarch can’t be married to a catholic. This legal ban doesn’t apply to any other faith – not Muslims, Jews, Hindus, nor to atheists," Clegg said.
"Times have changed, along with our attitudes towards each other. It is time for us to bring these arcane laws up to date."
Critics warn changing the rules could trigger a future constitutional crisis. For instance, if a future heir marries a Catholic, the rules of the Church insist their offspring should be raised a Catholic.
However, a Catholic would still not be able to sit on the throne, because the monarch also acts as head of the Church of England.
And of those rules were changed to allow Catholics on the throne, it would amount to the disestablishment of the church.
Clegg questioned whether "a blanket rule" on a Catholic upbringing really existed, to which Rees-Mogg cited a rule saying Catholics should make "best efforts" to bring up children in their faith, even in mixed marriages.
"Of course some Catholics fail but that doesn't mean there isn't a ruling," he added.
In response, Clegg emphasised that the rule stressed only the need for "best efforts" and pointed out the Catholic Church and the Church of England were united in supporting the bill.
In response, Soames said: "There is a presumption, if not an obligation, that children in Catholic mixed marriages should be brought up in the Catholic faith.
"Does the bill not make it more likely a Catholic could take the throne?"
He added: "These are not matters to be treated lightly. They touch upon the whole architecture of the laws and settlement of this country."
The legislative process the bill will have to pass through once it leaves the British parliament could also present difficulties for Whitehall.
The bill needs to win the approval of all 16 legislatures where the Queen is head of state, including tiny states with populations hovering among the 10,000 mark.
The requirement that all 16 parliaments verify the legislation could allow republican campaigners to sabotage the process, although it is not entirely clear what would happen if a state actually rejected the bill.
The ban on marrying Catholics goes back 300 years, when Britain was threatened by Louis XIV of France and other Catholic nations.