According to a note published on the official website of the Presidency of the Republic, the parties will be heard "following the Prime Minister's resignation and, in particular, also under the terms of Article 133º of the Constitution".
This constitutional rule states that it is up to the President of the Republic to dissolve the Assembly of the Republic, after hearing the parties represented in it and the Council of State.
The head of state will receive the parties at the Belém Palace in ascending order of parliamentary representation - Livre, PAN, BE, PCP, Iniciativa Liberal, Chega, PSD and PS - in meetings scheduled between 11:00 and 19:00.
Marcelo Rebelo de Sousa announced that he would "speak to the country immediately after the Council of State meeting".
The Council of State was convened "under article 145, paragraph a) and paragraph e), second part" of the Constitution - under which this body is responsible for "pronouncing on the dissolution of the Assembly of the Republic", but also, "in general, advising the President of the Republic in the exercise of his functions".
In the current legislature with an absolute PS majority, Marcelo Rebelo de Sousa said that a possible departure by António Costa would lead to the dissolution of parliament, ruling out the formation of another executive with the same majority.
António Costa submitted his resignation as Prime Minister to the President of the Republic on Tuesday after the Public Prosecutor's Office announced that he is the subject of an inquiry at the Supreme Court of Justice into lithium and hydrogen projects.
Declaring his "conscience clear", António Costa argued, however, that "the dignity of the functions of Prime Minister is not compatible with any suspicion of his integrity, his good behaviour or, much less, with the suspicion of any criminal act".
On Tuesday, searches were carried out in government offices, including the official residence of São Bento, targeting the prime minister's chief of staff, Vítor Escária, who was detained for questioning.
António Costa resigned after almost eight years as prime minister, a post he began on 26 November 2015.