Rishi Sunak was given early hope on Tuesday that the hardline Democratic Unionist Party may not torpedo his breakthrough post-Brexittrade deal for Northern Ireland.

With the Prime Minister in Belfast to start selling his agreement with the EU, DUP leader Sir Jeffrey Donaldson signalled cautious support for the “Stormont brake”, which gives Northern Ireland politicians a block on changes to Brussels laws which may affect NI trade.

The DUP’s view of the agreement, announced by Mr Sunak and European Commission President Ursula von der Leyen yesterday, is critical as to whether it will secure the backing of hardline Eurosceptic Tory MPs who have long argued that the Northern Ireland Protocol undermines UK sovereignty.

While some prominent Brexiteers including ex-Brexit Secretary David Davis and Northern Ireland minister Steve Baker have given the deal their support, former Prime Minister Boris Johnson has yet to declare his position.

The European Research Group of Tory backbenchers was due to meet today and will convene its “star chamber” of lawyers to scrutinise the deal.

Sir Jeffrey insisted the DUP still had some concerns and said the party would take time to consider the deal before deciding whether to back it. But he told BBC Radio 4’s Today programme: “I do think that what has been proposed at first reading does give Stormont the ability to apply the brake where the application of EU law for the purposes of facilitating cross border trade impacts on our ability to trade with the rest of the United Kingdom. If EU law or changes to EU law were to impact in a way that undermined our ability to trade within then I think it is right that Stormont has a brake will ultimately be able to veto any new such law.” The DUP is boycotting the National Assembly at Stormont until changes are made to the protocol, part of Mr Johnson’s 2019 Brexit Treaty which avoided a land border on the island of Ireland but effectively introduced a trade border in the Irish Sea.

Restoring power sharing at Stormont will be seen as a major win for Mr Sunak’s premiership and will be warmly received by US President Joe Biden, who is expected to travel to Northern Ireland in April for events to mark the 25th anniversary of the Good Friday peace agreement.

While Sir Jeffrey struck a cautiously positive tone, other prominent figures in the DUP have already criticised the deal with MP Ian Paisley insisting it didn’t “cut the mustard”.

Speaking on Tuesday morning, Mr Sunak described the Windsor Framework as a “huge step forward”.

Rishi Sunak speaks to business leaders in Lisburn, Northern Ireland

He told the Today programme: “I have spent a lot of time listening to Unionist communities from Northern Ireland and indeed all parties that I’ve engaged with, because this is about everybody, and I have taken the time to understand their concerns.

“I am confident that the Windsor Framework addresses those concerns but I also respect that everybody, including Unionist representatives of all parties, will need the time and the space to consider the detail.”

The Prime Minister also seemed to suggest he has spoken to Mr Johnson but added: “It is not about personalities, it is not about Westminster. This is about the people of Northern Ireland and what is best for them”. The Windsor Framework removes barriers on trade across the Irish Sea with goods bound for NI from GB now able to travel through a new green lane with no physical checks except for those to prevent smuggling or crime. Meanwhile, goods heading on to Ireland and the EU market are subject to EU checks in a red lane.

The deal also ensures Northern Ireland benefits from the same tax policies as the rest of the UK, simplifies the process of sending parcels, medicines and chilled meats from Great Britain to the region and allows pets to travel more smoothly with their owners across the whole of the country.

But it still includes a role for the European Court of Justice and will mean the region still follows some EU single market rules in limited areas.

Sir Jeffrey added: “This is a very complex agreement. We only received it yesterday afternoon so we’re going to take time to study the legal text to get legal advice on it. And then we’ll come to a conclusion on the agreement as a whole.”

It came as the Foreign Secretary James Cleverly rejected claims that the King was being drawn into political controversy over his meeting with European Commission president Ursula von der Leyen. He denied ministers were politicising the monarchy following the meeting between the King and the Brussels official at Windsor Castle yesterday, which coincided with the agreement of the deal for Northern Ireland.

Mr Cleverly insisted it was “not unusual” for the King to meet senior international figures while they were in the UK. He pointed out that he had met Volodymyr Zelensky earlier this month, although the Ukrainian President is a head of state, unlike Ms von der Leyen.

Mr Cleverly told LBC radio: “It was our invitation for Ursula von der Leyen to come to the UK to finalise this deal with the Prime Minister. Of course, that was a conversation we had with the Palace.

“The final decision on the availability of His Majesty is with the Palace.”

Asked who arranged Ms von der Leyen’s meeting with the King, Mr Cleverly told Sky News: “Decisions about the King’s diary are, rightly, for the Palace.”

Buckingham Palace said Charles was acting on “the Government’s advice”. Downing Street said it was “fundamentally” a decision for the King.

Government sources indicated Ms von der Leyen had requested the meeting, something denied in Brussels.

Leading Brexiteer Jacob Rees-Mogg said it was wrong to involve the King in the “immediate political controversy” on the day the PM signed a new agreement with Ms von der Leyen.