Boris Johnson resigns: Northern Ireland politicians react to the Prime Minister's resignation
Boris Johnson's replacement will "face very significant challenges both at home and abroad", the DUP leader has warned.
Sir Jeffrey Donaldson made the comments after the Prime Minister announced his resignation on Thursday.
With a huge number of resignations and incoming ministers being appointed, Sir Jeffrey recognised the last 24 hours had been "unprecedented".
Despite the changes in government, he insists the Northern Ireland Protocol is "still undermining the foundations of devolved government in Stormont and must be replaced."
Sir Jeffrey said: "It is no secret that we believed that Boris Johnson had a duty to get rid of the Irish Sea border having disastrously gone against our advice and signed a Withdrawal Agreement containing the protocol.
"After two years of failed negotiations, the efforts to remove the protocol must continue.
"Stable devolved government can only be built on a solid foundation. With zero unionist support in the Assembly, the NI Protocol has been undermining the foundations rather than underpinning them.
"The leadership of the Conservative and Unionist Party is a matter for the MPs and members of that Party. They should, however, recognise that no unionist MLAs or MPs support the protocol. Fully functioning devolved government in Stormont and the protocol cannot co-exist."
Meanwhile, TUV leader Jim Allister blasted Mr Johnson's tenure as "disastrous" for unionism, "with the introduction of the protocol ensuring that far from securing Brexit for the entire nation he has left Northern Ireland subject to EU writ and partition from the rest (of the) UK".
He added: "With Westminster in state of flux, it is more imperative than ever that Unionism holds firm in using Stormont leverage against the iniquitous protocol."
Sinn Fein leader Mary Lou McDonald responded to the news of Mr Johnson's resignation by criticising his leadership, and stressing the need to implement the Good Friday Agreement "in full".
She said: "My sole concern at this point is that whomever leads the British Government, that they now enter into full compliance with international law and work to implement in full the Good Friday Agreement.
"Boris Johnson's sole concern in recent times has been to placate the DUP, and to facilitate the most negative, reactionary strain of political unionism, all in a vein, glorious attempt to hold on to power himself. It's always been about him.
"We need leadership that is about fulfilling international commitments and in the case of Ireland a government that understands they cannot and will not inflict damage on Ireland."
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