Austen Morgan (pictured) said such an act would breach a clause in the Good Friday agreement that promotes parity of esteem between all cultures in Northern Ireland.

Article one, paragraph three of the agreement provides: “All participants recognise the importance of respect, understanding and tolerance in relation to linguistic diversity, including in Northern Ireland, the Irish language, Ulster-Scots and the languages of the various ethnic communities, all of which are part of the cultural wealth of the island of Ireland.”

Mr Morgan, who was legal adviser to David Trimble in the lead up to the agreement, said a “respectable legal challenge” was possible if an exclusionary act was brought in.

Mr Morgan added: “An Irish language act would not be per se contrary to. However, in the Sinn Féin/DUP context, where the latter offered a culture act, it would appear that the advancement of Irish was contrary to other languages.

“ binds ‘all participants’ in the talks. That included Sinn Féin. However, while Sinn Féin could argue that it never signed up to the Belfast agreement, it has sought to give that impression, so would never argue that it was not bound.

“The challenge could be to a Northern Ireland department (culture) or the secretary of state, to prevent them acceding to Sinn Féin’s demand when the DUP offer is on the table. An applicant could seek a declaration that it was contrary to the Belfast agreement.”