DUP leader Sir Jeffrey Donaldson is refusing to nominate ministers to a new Stormont Executive without action on the protocol.
Efforts to remove the Northern Ireland Protocol must continue in the next government, Sir Jeffrey Donaldson has said.
Mr Johnson is set to stay on as premier until a new Conservative leader is elected by the party.
With the race to become the next leader unofficially under way, Sir Jeffrey emphasised that the Brexit protocol is “undermining the foundations of devolved government in Stormont”, insisting it “must be replaced”.
The Stormont Assembly has been collapsed for several months while the DUP refuses to nominate ministers to a new Executive until the UK government takes action on the protocol.
Unionists oppose the post-Brexit trading arrangements which demand checks on goods arriving into Northern Ireland from Great Britain, condemning it as a border in the Irish Sea.
Sir Jeffrey said the incoming leader of the Conservative Party and next prime minister “will face very significant challenges both at home and abroad”.
“I recognise the efforts made by the outgoing Prime Minister Boris Johnson and his team to replace the Northern Ireland Protocol with arrangements which can command the support of unionists as well as nationalists,” he 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 coexist.”
TUV leader Jim Allister blasted Mr Johnson as Prime Minister 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 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.”