Gerry Adams remains MP for West Belfast despite dispatching a resignation letter to Westminster, according to the Office of the Speaker of the House of Commons.
The Sinn Féin leader's decision to give up his seat has set republican disregard for parliamentary procedures on a collision course with the requirements of constitutional formality.
Adams announced in November that he would be standing for Sinn Féin in the Louth constituency in the Irish Republic's forthcoming general election. That contest is now expected next month.
He also declared that he did not want to be a member of both the Dáil in Dublin and the House of Commons in Westminster, although there is nothing legally to disbar him from dual membership.
Believing that another Sinn Féin candidate should stand for the West Belfast seat and take up the constituency work, Adams last Friday wrote to the Speaker, John Bercow, formally informing him of his departure.
But under parliamentary rules dating back to 1624, an MP who wishes to quit has to apply for one of a number of obscure, paid crown posts: Crown Steward and Bailiff of the three Chiltern Hundreds of Stoke, Desborough and Burnham or Steward of the Manor of Northstead.
A Sinn Féin spokesman in Belfast said: "We couldn't give a toss [about these rules]. He's not going to apply for these offices. He has sent in a resignation letter like any ordinary person. We want a byelection in West Belfast. There's no written constitution; they just make it up anyway. It's strange men who parade around in tights. Republicans are not losing any sleep over this."
The parliamentary regulation is enshrined in 400 years of constitutional history and may be difficult to overturn without setting a political precedent.
According to Erskine May, the bible of parliamentary procedure, it is "a settled principle of parliamentary law that a Member, after he is duly chosen, cannot relinquish his seat; and, in order to evade this restriction, a Member who wishes to retire accepts office under the Crown which legally vacates his seat and obliges the House to order a new writ".
The Speaker's office said there was one alternative route - or piece of political pantomime - open to Sinn Féin MPs, who have refused to take the oath of allegiance to the crown, as well as their seats in Westminster.
If an elected MP who has not sworn the oath attempts to take his or her seat in the Commons during a debate, then that would also formally lead to their ejection from Westminster.
Under the Parliamentary Oaths Act 1865, "if the MP sits in the house without having taken the oath" then he would lose his seat, the Speaker's Office confirmed. Sinn Féin said Adams would not participate in such a political charade.
