A 'line of succession' of senior politicians who would replace an incapacitated Prime Minister places the Northern Ireland Secretary at the very bottom.
An MP has said that such a list should be guaranteed in law, amid concerns for the health of Boris Johnson.
Conservative Peter Bone welcomed the Government's foresight to have Foreign Secretary Dominic Raab as Mr Johnson's formal deputy. But the MP for Wellingborough said it is unclear what would happen if Mr Raab should also fall ill, adding there is a need for legal certainty.
Mr Bone's Prime Minister (Temporary Replacement) Bill could be debated in the Commons this summer and he hopes MPs will support his proposal, which he has moved several times previously.
Mr Bone said: "The first thing I'd want to say is my thoughts and prayers, and I'm sure those of my constituents, are for Boris and Carrie and hoping they will make a speedy recovery.
"The second point is there had been an ad hoc arrangement put in place so that Dominic Raab became effectively prime minister, he's effectively prime minister at the moment, and I'm pleased that happened so there isn't an immediate crisis.
"But what my Bill tries to do is set out in law the succession - it could have said Foreign Secretary, Chancellor, and so on - as there has to be a question mark if Dominic Raab was to fall ill, who would take over? It's to give certainty to the situation and it is what it says, a temporary prime minister replacement."
A previous version of Mr Bone's Bill, then known as the Prime Minister (Replacement) Bill, included a succession list.
This read: deputy prime minister, home secretary, defence secretary, foreign secretary, the chancellor, transport secretary, health secretary, business secretary, justice secretary, communities secretary, education secretary, environment secretary, work and pensions secretary, Cabinet Office minister, paymaster general, culture secretary, attorney general, energy secretary, international development secretary, Commons leader, Lords leader, Scottish secretary, Welsh secretary, Northern Ireland secretary.