ANTI-Brexit campaigners have launched a new legal challenge in Scotland to declare Boris Johnson's new Brexit deal with the EU unlawful.

Campaigner Jo Maugham QC, who was behind the bid to declare the successful bid to have the Prime Minister's proroguing of Parliament declared unlawful, is behind the new bid.

He has confirmed that a petition had been lodged at the Court of Session and is expected to be heard on Friday.

He is arguing that the new deal contravenes legislation that prevents Northern Ireland forming part of a separate customs territory to the rest of the UK.

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Section 55 of the Taxation (Cross-border Trade) Act 2018 prevents Northern Ireland from having different customs rules than the rest of the UK.

The clause titled 'Single United Kingdom customs territory' states: "It shall be unlawful for Her Majesty's Government to enter into arrangements under which Northern Ireland forms part of a separate customs territory to Great Britain."

And he is arguing any agreement which makes arrangements that puts Northern Ireland in a separate customs territory to Great Britain "cannot, lawfully" be laid before the Houses of Parliament.

The challenge is due to heard at 10am in the Outer House of the Court of Session before Lord Pentland.

The Democratic Unionist Party has said it will not support the new Brexit deal because it says it "undermines the integrity of the Union".

It says Northern Ireland's main route of trade on an east-west basis will be subject to rules of the European Union Customs Union, even thought it will reamin part of the UK Customs territory.

"All goods would be subject to a customs check regime regardless of their final destination," the DUP said.

"The default position, even for goods travelling from one part of our country to another, is that they are considered under the EU Customs code unless otherwise agreed.

"We recognised that only those goods ultimately destined for the Republic of Ireland would be subject to tariffs but the reality remains that the EU would have a veto on which goods would be exempt and which would not...

"This is not acceptable within the internal borders of the United Kingdom."

Mr Maugham claims that if the court finds the proposed agreement is unlawful Boris Johnson would be obliged to request an extension to Brexit negotiations, under the terms of the Benn Act which states that s the prime minister must ask the EU for a delay if Parliament does not agree a deal by Saturday.

The petition says that Section 44 is "clear in its terms" and that the government has "no discretion to decide not to follow its terms".

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It says that defying the prohibitions set out in Section 55 would result in a "substantial and irreversible change" to the UK's constitution effected by "the carrying out of unlawful acts on the part of the UK government".

The petition says that Section 55 was derived from a backbench amendment sponsored by the European Research Group of Conservative MPs.

"The avowed intent of the promoters of that provision was to prevent the government from entering into any 'Northern Ireland backstop' arrangement under which, post-Brexit, the territory of Northern Ireland would continue to be associated with the customs territory of the European Union such as to avoid the need for any customs control between Northern Ireland and the EU...."

After details emerged of the Brexit deal, Mr Maugham said: "We do not understand how the government might have come to negotiate a withdrawal agreement in terms that breach amendments tabled by its own European Research Group.

"Unless and until Section 55 is repealed by the UK Parliament, it is simply not open, as a matter of law, for the United Kingdom to enter into such an agreement.

"If the proposed withdrawal agreement is unlawful, the government will be obliged to request an extension as mandated by the Benn Act and in accordance with undertakings given to the Court of Session in Vince, Maugham, Cherry v Boris Johnson."