Northern Ireland has been “partly annexed by the EU” through the workings of the NI Protocol, a court challenge to its legality has been told.

A judicial review on the protocol, part of the Brexit deal that creates a trade border between Northern Ireland and Great Britain, has begun at the High Court in Belfast.

The judicial review is being taken in the name of unionists across the UK, including outgoing DUP leader Arlene Foster, outgoing UUP leader Steve Aiken, TUV leader Jim Allister and Belfast Agreement architect Lord Trimble.

Launching the challenge on Friday, John Larkin QC said the judge, Mr Justice Colton, would be asked to take a view on the legality of the protocol.

He argued that the protocol is unlawful because it breaches the 1800 Acts of Union which created the UK, and the 1998 Good Friday Agreement.

Mr Larkin, a former attorney general in Northern Ireland, referred to an affidavit from former Ulster Unionist leader Lord Trimble, as part of his argument that the protocol is not consistent with the principle of consent enshrined in the Good Friday Agreement.

Reading Lord Trimble’s words, he said: “The issue of consent is a hugely important part of the Belfast Agreement, it is one of the key pillars underpinning the entirety of that agreement. The protocol fundamentally changes the relationship between Northern Ireland and the rest of the UK.

“Laws which will apply to the economy, environment, agriculture, workers’ rights and regulations covering everything from building standards to the use of weedkillers, these will no longer be made in our parliament at Westminster or the local Assembly, they will be determined by a foreign authority in Brussels.

“This amounts of tens of thousands of separate regulations which no-one in the UK or Northern Ireland will have any capability to vote on or discuss.”

Still reading from Lord Trimble’s affidavit, Mr Larkin continued: “This amounts to a seismic change in the constitutional position of Northern Ireland. The central pillar of the agreement is the principle of democratic consent to any change in the constitutional arrangements of Northern Ireland.

“We endured 35 years of a terrorist campaign, terrorists did not succeed. The main object of the Belfast Agreement was to bring that terrorism to an end and present a democratic way forward.

“The protocol ignores the principle of consent. Northern Ireland is no longer fully part of the UK, it has been partly annexed by the EU.”

Mr Larkin said that Lord Trimble had made huge personal and political sacrifices to bring about the Good Friday Agreement and said the former UUP leader felt “personally betrayed” by the protocol.

During the hearing, Mr Larkin focused on separate articles of the protocol which related to trade, taxation, the movement of goods and regulatory checks coming into Northern Ireland.

He said: “The so-called unfettered access is not unfettered.

“The constitutional importance of this is enormous. Goods coming from Northern Ireland simply cannot be described in the same way as a good coming from Liverpool, or Edinbugh, or Cardiff or London.

“Northern Ireland is subject to taxation without representation.

“It can be likened to the position of the Vichy regime which was relied on to do the bidding of the occupiers; the occupiers were free to inspect or deny things, but when push came to shove the occupiers gave an instruction and the Vichy authorities would quickly fall into line.”

The Vichy regime administered France during Nazi occupation.

The former attorney general began the day by arguing that the protocol is “unlawful on a number of grounds” and incompatible with the 1800 Acts of Union.

He said: “Article 6 of the Acts of Union prevented and prevents Her Majesty’s Government from agreeing any treaty of provision with a foreign power or implementing any such provision which places Her Majesty’s subjects of Northern Ireland and Her Majesty’s subjects of Great Britain on a separate footing in relation to that foreign power.

“Contrary to that, the protocol, in its entirely, places Her Majesty’s subjects in Northern Ireland on a different footing to Her Majesty’s subjects in Great Britain in relation to the European Union and the protocol.

“Article 6 of the Acts of Union prevented and prevents Her Majesty’s Government from agreeing any treaty provision with a foreign power implementing a domestic law and any such provision which conflicts with the obligation in that article that Her Majesty’s subjects in NI and GB should have the same privileges and should be on the same footing.”

Mr Larkin argued that the Government could have amended the Acts of Union, but chose not to do so.

He continued: “Article 6 of the Acts of Union imposes certain limitations to make treaties and those were ignored.

“HMG breached Article 6 by concluding an agreement. Parliament will be presumed to be acting with a view to legality and not giving effect to an unlawful treaty.

“If Parliament wishes to take the constitutional risk it can … It hasn’t done that.

“We have a partly written constitution in the United Kingdom, and part of our written constitution are the Acts of Union and they are certainly to be taken seriously.

“Ask yourself this question: did the legislature have the opportunity to put the matter clearly? It most certainly did. Did it take that opportunity? It most certainly did not.

“What is the consequence of that? The consequence is that there has been no repeal or amendment (of the Act of Union).”

The full list of applicants in the case includes Ben Habib and Baroness Hoey as well as Lord Trimble, Mrs Foster, Mr Aiken and Mr Allister.

The case will resume on Monday and is scheduled to last for four days with judgment to be delivered later in the year.