Mr Puigdemont, of the platform Junts Per Catalunya, and Martina Rovira of the Republican Left (ERC) hammered out the agreement at a dinner on Tuesday night in the Belgian capital, where he fled in November to avoid arrest on sedition and rebellion charges.
The plan may offer a way out of the chaos surrounding the formation of a new Catalan government at a time when many of its senior candidates are either in jail or facing prosecution. The two options – which would either see Mr Puigdemont attending the Parliament on a video call or a delegate reading the inauguration speech on his behalf – are currently being studied by lawyers.
It is understood that the parties believe neither method to be specifically prohibited under parliamentary rules. But they would almost certainly face a legal challenge from the Spanish government or other political opponents.
Fernando Martínez Maillo, general coordinator of the ruling Popular Party, said it was ready to “impede by any means at our disposal” a long-distance inauguration of Mr Puigdemont. Insisting that such methods would be illegal, Mr Maillo said anything other than a traditional investiture – in which a president presented himself and his programme to parliament for a vote – would be “a real mockery, first of the Catalans, and then of the rest of the parliamentary groups”.