Members of the Protestant Orange Order have descended on Downing Street to oppose the lifting of a ban on those in line to the throne from marrying a Catholic.
The Orangemen took a brief detour from their annual Remembrance Day march to deliver the letter setting out concerns that changes to the Act of Settlement will call into question the future role of the monarch as the Supreme Governor of the Church of England.
Jeffrey Donaldson, Ulster Democratic Unionist MP for Lagan Valley, said: "The Queen is the supreme governor of the Church of England, and if a future monarch were to marry someone outside of the Church of England that could have consequences for the sole link between the Crown and the church.
"We believe this is part of a long-term plan to disestablish the Church of England by damaging the link between the Crown and the church."
The leaders of the 16 Commonwealth nations where the Queen serves as head of state came together in October to unanimously approve changes to the laws of royal succession which lift the ban on anyone in line for the throne marrying a Catholic.
The Orangemen Remember and Parade in London started with the laying of a wreath at the tomb of King William III in Westminster Abbey by nine officers and members of the Houses of Parliament Loyal Orange Lodge.
Prayers were said and Mr Donaldson, the 48-year-old Master of the Houses of Parliament Loyal Orange Lodge, addressed the group.
He said: "We urge upon our government today to think carefully about the course they are embarked upon and its consequences for our sovereign, for our church and for our nation."
The parade featured around 100 members dressed in black suits with ceremonial orange and purple collarettes and white gloves.
They were accompanied by the Corby Purple Star Flute Band from Northampton, who played traditional marching band songs.