Northern Ireland's government has been plunged into chaos after First Minister Paul Givan resigned in a row over Brexit.
Power-sharing arrangements in Northern Ireland mean that Deputy First Minister Michelle O'Neill automatically loses her position.
Other ministers can still remain in place but the government will not be able take any significant decisions, as the Northern Ireland Executive is chaired by the First and Deputy ministers.
Mr Givan told reporters: "When I first entered the Assembly 12 years ago, I never expected to have the opportunity to lead the Government and serve the people of Northern Ireland as First Minister.
"Holding this office is one that comes with a heavy responsibility and I have often felt the weight of this burden, to do what is right for all our people."
Northern Ireland Secretary Brandon Lewis said the decision was "extremely disappointing" and urged the DUP to reinstate the First Minister immediately.
He said: "I hope that Northern Ireland’s political leaders will take the necessary steps to restore the stability in the devolved institutions that the people of Northern Ireland deserve."
It comes amid escalating tensions over a key part of Boris Johnson's Brexit deal, which created checks on goods travelling into Northern Ireland from Great Britain.
DUP minister Edwin Poots ordered officials to stop checks on food and farming goods at midnight on Thursday - which are required under the Brexit agreement.
But checks appeared to be continuing on Thursday morning, with lorries being received at Belfast Port.
Boris Johnson branded it "crazy" to "have checks on goods that are basically circulating within the single market of the United Kingdom" - despite negotiating them himself.
Speaking on a visit to Blackpool, he said: "What you could have, of course, is a common-sensical... practical steps to weed out, to check on things that might be at risk of circulation, as they say in Brussels, are at risk of circulation in Ireland, as well as Northern Ireland.
"Now we can do that, but without having a full panoply of checks on the GB/NI coast and at the airport, and that's the way forward.
"I think practical common sense is what's needed."
The European Commission said the move creates "further uncertainty and unpredictability" - and political rivals Sinn Fein have branded it a political stunt.
Environment Secretary George Eustice told MPs that agri-food checks are a "devolved matter" and it was "entirely unnecessary" for the UK Government to intervene.
Speaking in the Commons, he said: "The bar for such an intervention is high, and rightly so, and it's also entirely unnecessary at this stage.
"The checks are actually continuing, there is no change at the moment. Yes, a direction has been issued, officials in Daera (Department of Agriculture, Environment and Rural Affairs) are taking their own legal advice as accounting officers on elements related to that.
"What we very much hope is that in the first instance the implementation of this can indeed be delivered in its right and proper place through the Northern Ireland Executive."
Labour accused the UK Government of acting as "bystanders" while their Brexit deal falls apart.
Shadow Northern Ireland secretary Peter Kyle told the Commons: "In the last week, both the Foreign and Northern Ireland Secretaries said the Irish Sea border checks are a matter for the Northern Ireland Executive.
"The protocol was signed into international law by the UK Government, and now they are bystanders as their deal falls apart - pathetically claiming it's all someone else's responsibility.
"Just think of the implications: Is the message that the Welsh Senedd or Scottish Parliament can break international law too and the Government will have nothing to say about it?
"It's another piece of vandalism committed against our union by a reckless Government too busy partying to notice what's going on in the real world."
The Northern Ireland Protocol - which the UK agreed with the EU - aimed to prevent a return to violence by avoiding a land border between Northern Ireland and the Republic of Island.
Instead, a trade border was effectively created in the Irish Sea.
The Prime Minister has since tried to tear up the deal to reduce border checks, which have caused red tape and delays for businesses and consumers in Northern Ireland.
Foreign Secretary Liz Truss has taken over the negotiations following Brexit Minister Lord Frost resignation.
The DUP oppose the NI protocol over fears Northern Ireland is being treated differently to the rest of the UK.
A European Commission spokesperson said the decision was "unhelpful" and "creates further uncertainty and unpredictability for businesses and citizens in Northern Ireland"
"The European Commission will closely monitor developments in Northern Ireland pursuant to this announcement," the spokesperson added.
"Vice-president Maros Sefcovic will speak to the UK Foreign Minister Liz Truss on Thursday afternoon to continue our discussions on finding durable solutions for the people of Northern Ireland.
"He will recall that controls on goods arriving in Northern Ireland from Great Britain are a key element of the protocol."
