Arlene Foster formally resigned as first minister on Monday, criticising the "imbalance" of the Northern Ireland protocol, a key measure in the Brexit agreement, in her farewell speech.
"An imbalance and an instability is built in [to the protocol] that will fester and deteriorate, and if Brussels continue to think that the protocol is enough, they are in denial," Foster, who resigned amid tensions over Brexit, said.
"Imbalance and instability in the context of Northern Ireland is a truly a dangerous cocktail.
"Northern Ireland is part of the United Kingdom and needs to be treated as such and if the EU does not, then the UK will have a practical, political, and moral obligation to act to protect the everyday life of everyone in Northern Ireland," Foster added.
The protocol, designed to maintain an open border on the island of Ireland, means there are some checks between goods moving between Northern Ireland and the United Kingdom as part of the post-Brexit agreement.
Tensions, in part over Brexit, had led to violence in the streets of Belfast earlier in the year.
Foster, meanwhile, announced in April that she would resign as Democratic Unionist Party (DUP) party leader and first minister amid mounting pressure in her party over her support of the Brexit deal.
Foster first became DUP leader in 2015 and first served as First Minister in 2016.
A new DUP party leader, Edwin Poots, was elected last month.
Now Poots has picked Paul Givan, a 39-year-old lawmaker and former communities minister, to become the next first minister of Northern Ireland.
But the Northern Ireland executive is in a power-sharing relationship with Irish nationalist party Sinn Fein that was restored in January 2020 after three years of deadlock.
It means Sinn Fein's Michelle O'Neill, deputy first minister, will also have to be re-nominated for her position after Foster's resignation.
Following Foster's resignation, effective on Monday, there's a period of seven days for parties to agree on a new first minister from the DUP and deputy first minister from Sinn Fein.
Both Poots and Givan are viewed as part of the more conservative wing of the party and the appointment of Givan has reportedly caused some concern about coming to an agreement with Sinn Fein.
Sinn Fein supports legislation for the Irish language while Givan controversially cut funding for the Irish language in 2017.
The agreement that ended the stalemate in the Northern Irish assembly in 2020 included measures to protect the Irish language.
Some are concerned that the new political shakeup could lead to further stalemate in the country.