Speaking after a meeting with Mr Johnson at Hillsborough Castle, Sinn Fein president Mary Lou McDonald said: “We find ourselves in a situation where the prime minister is quite willing to act in coordination with the DUP.”
She added that it was “not acceptable”.
Ms McDonald said it appears the UK Government’s priority is “placating the DUP”.
She said: “It’s very clear to us that despite all of the rhetoric from the British Government about re-establishing the Executive here in the north, that in fact their priority is placating the DUP.
“We’ve had what we would describe as a fairly tough meeting with the Prime Minister.
“We have put it to him very directly that the absolute priority is getting government working here in the north.
“People are facing incredible difficulties in the midst of a cost-of-living crisis, and it’s simply not acceptable, it’s not good enough for anybody, the DUP or the British Government, to hold society here to ransom.”
It comes as tension between London and Brussels are intensifying over the prospect of Mr Johnson using domestic legislation at Westminster to nullify parts of the Brexit Withdrawal Agreement that require checks on goods moving between Great Britain and Northern Ireland.
Foreign Secretary Liz Truss is expected to formally announce a plan to legislate on the protocol on Tuesday, although an actual parliamentary Bill is not expected to be published at that point.
Mr Johnson is conducting emergency talks with Stormont’s political leaders in a bid to break a deadlock linked to the protocol.
The power-sharing institutions in Belfast have been plunged into crisis in the wake of the recent Assembly election, with the DUP refusing to re-enter a devolved government in protest at trading arrangements the party claims are undermining the union.