Sinn Féin confirmed it had lodged a motion of no confidence in the Assembly's DUP speaker, emphasising the fraying of relations between Stormont's two main parties,
Robin Newton has been under political pressure over his handling of a recalled Assembly session to debate RHI on December 19 as well as his relationship with a controversial charity in his east Belfast constituency.
Sinn Fein chief whip Caral Ni Chuilin said: "The performance in the Assembly on 19th December was a shambles and totally compromised the independence and integrity of that office.
"The speaker should be above reproach and independently accountable to the political institutions.
"However, he is acting without regard to the integrity of the office and as such his position is now untenable.
"He is now part of the problem and an integral part of the unfolding crisis and he should resign immediately."
Mr Newton cited "procedural advice" in permitting Mrs Foster to address the chamber even though Mr McGuinness had withdrawn his approval for a statement that was initially supposedly to be made on behalf of them both.
Opposition had parties walked out of the chamber in protest.
The speaker has also been forced to defend himself against conflict of interest accusations in regard to his handling of Assembly exchanges on charity Charter NI.
He has already apologised to MLAs for not declaring that he had offered advice to Charter NI during constituency work before ruling against an urgent debate on its funding.
The DUP did not respond to a request for comment on the motion.
