Jeffrey Donaldson can not give assurance that Paul Givan will be First Minister next week, he warned today.
Comments from the DUP leader come as he met with the Prime Minister on Wednesday to discuss the Northern Ireland Protocol.
Speaking outside Downing Street, Mr Donaldson warned that his party will "take the action we said we would take" if there was no agreement.
"I can't give that assurance [on Paul Givan remaining First Minister next week] because that depends on what others will do," he told UTV.
"But we are very clear and this week we are taking steps in relation to the checks that we believe are unacceptable, that are taking place at Northern Ireland ports.
"The Executive will need to make a decision on those checks. In the absence of that decision, then we will take appropriate action.
"But secondly, I also am clear that if the government are not prepared to move, if there is not agreement between the UK and the EU government on removing the barriers to trade within the UK, and if the government is not prepared to trigger Article 16 in the absence of such agreement, then we will take the action that we said we would take."
Sinn Féin MP John Finucane said threats by Jeffrey Donaldson to walk away from the Assembly and Executive are "reckless and irresponsible".
Mr Finucane said: "To threaten instability at a time when the health service and the Executive continues to deal with the challenges of the pandemic is reckless and irresponsible."
