Sir Jeffrey Donaldson, the party’s leader, told the Prime Minister that fixing the Northern Ireland Protocol “isn’t just a question” of scrapping Brussels red tape.
Speaking after they met at a Belfast hotel, he said that Number 10 must “honour the commitments” it made to restore the province’s status in the Union.
However, he adopted a neutral tone and repeatedly refused to criticise reported details of the agreement, which will encourage Downing Street.
His talks with Sir Jeffrey, scheduled for just 15 minutes, ended up lasting more than an hour.
Afterwards, the DUP leader suggested that Number 10 faces an uphill task to persuade Unionists to end their boycott of the Stormont Assembly.
He warned Mr Sunak that his party will not be bounced into signing anything, adding: “I do not believe anyone should be led by a calendar.
“The decisions that will be taken by the Prime Minister and the European Commission will either consign Northern Ireland to more division or they will clear a path towards healing and the restoration of the political institutions.
“I’ve indicated to the Prime Minister that it’s fundamentally important he agrees the right deal. I want to hear that Brussels will stretch itself to recognise the concerns we have as unionists and that this process will correct the wrongs of the last negotiations.”
Asked where the DUP might be prepared to meet the Government in the middle, he replied: “It’s not a question of us compromising.
“It’s a question of the UK Government honouring the commitments they’ve made to the people of Northern Ireland, delivering on those commitments in the negotiations with the EU and providing the basis upon which Northern Ireland’s place within the UK and its internal market can be respected and protected.”
Sir Jeffrey warned Downing Street that a deal which focuses solely on reducing red tape in the Irish Sea is unlikely to win his party’s support.
“This isn’t just a question of checks and bureaucracy. It’s a question of whether we have the ability to trade freely within the UK and its internal market,” he said.
“When we trade within the UK, then we should follow UK standards and UK rules. That is our clear position. That is in essence what we need to get as an outcome from this negotiation.”
Under the Protocol, which came into force in Jan 2021, Northern Ireland effectively stayed in the EU single market and continues to follow EU rules.
The fix avoided creating a hard border with the Republic of Ireland, but only at the expense of moving checks on goods into the Irish Sea instead.
Unionists say that the new red tape between Great Britain and Northern Ireland has undermined their place in the UK and damaged the economy.
They have also been angered by the fact that the European Court of Justice (ECJ) polices the application of EU rules, over which Belfast has no say.
‘There’s a way to go yet’
It is understood that the deal set to be struck with Brussels will eliminate the vast majority of checks on goods by creating green and red lanes.
Products destined exclusively for the Northern Irish market would be exempted from almost all controls, significantly easing the burden on businesses.
The ECJ would be put at arm’s length, with disputes having to go via the Northern Irish courts or an independent panel before being referred to it.
But crucially, Luxembourg-based judges will still retain their power as the “ultimate arbiter” of European law and their rulings would remain binding.
Other party leaders who met Mr Sunak said that he had told them while progress was being made towards a deal, “there’s a way to go yet”.