DUP leader Jeffrey Donaldson today threatened to stop all goods checks in Northern Ireland if Boris Johnson does not hold firm in the protocol row with the EU.
Sir Jeffrey warned that his party is 'not going to be in the business' of implementing the divorce terms unless it believes they are right for the province.
He also suggested the unionists could suspend key parts of the peace agreement if a compromise could not be reached.
The intervention, in an interview with The Times, came after Ursula von der Leyen slapped down the PM's demand for the EU to 'look seriously' at UK proposals for overhauling the protocol.
The European Commission president said the part of the Brexit agreement that has brought social and economic strife to Ulster could not be re-written.
In a nod to resentment at the way Mr Johnson handled the original deal with Brussels, Sir Jeffrey - who is expected to become First Minister when a seat comes up at Stormont - warned he would not base his policy on 'trust alone'.
'We have to take our own view on these things,' he said.
Sir Jeffrey Donaldson warned that his party is 'not going to be in the business' of implementing the divorce terms unless it believes they are right for the province
Boris Johnson (right) and Lord Frost (left) have laid out proposals for overhauling the Brexit rules for Northern Ireland
Sir Jeffrey said his party had set out seven tests for approving any new arrangements, to ensure that they did not impose 'unacceptable' barriers between Northern Ireland and the mainland UK.
'Those seven tests are all based on commitments that the government has made itself,' he said. 'If the government doesn't live up to those commitments and if the outcome fails to remove the Irish Sea border then ... there is no way we can work and co-operate on a North South basis when the Irish government and the EU are undermining our relationship with Great Britain.'
Earlier this week ministers revealed a new plan to ease the deadlock over checks on goods including chilled meats to Northern Ireland.
Brexit minister Lord Frost said on Wednesday that the Protocol, part of the Brexit divorce deal agreed by the UK and Brussels, was undermining the Good Friday Agreement and urged the European Union to look at UK proposals for change.
Downing Street said the Prime Minister told Ms von der Leyen in a call to engage with solutions put forward by the UK because 'solutions could not be found through the existing mechanisms of the Protocol'.
Ms von der Leyen tweeted: ''The EU will continue to be creative and flexible within the Protocol framework. But we will not renegotiate.
Protests against the Northern Ireland protocol termsin Belfast earlier this month
'That was why we had set out proposals for significant changes to it. He urged the EU to look at those proposals seriously and work with the UK on them.
'There is a huge opportunity to find reasonable, practical solutions to the difficulties facing people and businesses in Northern Ireland, and thereby to put the relationship between the UK and the EU on a better footing. They agreed to remain in touch.'
But after the call Ms von der Leyen tweeted: 'The EU will continue to be creative and flexible within the Protocol framework. But we will not renegotiate.
'We must jointly ensure stability and predictability in Northern Ireland.'
The protocol was put in place to prevent a hard border with Ireland, but it has instead effectively placed a trade barrier in the Irish Sea.
Lord Frost said the economic and social damage caused by the arrangements would have justified the use of Article 16, effectively tearing up parts of the deal.
Lord Frost's proposals are thought to require changes to at least three of the protocol's articles.
He called for a 'standstill' period, preserving the current grace periods and suspending legal action taken by the EU against the UK while changes are negotiated.
Mr Johnson also made similar pleas in a call with German Chancellor Angela Merkel