The Prime Minister is the head of the Government. In general, he forms the Government of which he leads and coordinates the activities. He chairs the Council of Ministers, and therefore has real authority over his colleagues. His dismissal generally also means that his Government is dismissed.
However, this position, which is clearly of essential importance in the political system of our country, does not have any special written legal status. Originally, it was not even mentioned in the Belgian Constitution.
The Governments formed from 1830 were given the name of the minister responsible for forming them, but his priority was not actually guaranteed, in particular, because the King himself often chaired the Council of Ministers from 1831. However, over time, the function of the person appointed as "Head of the Cabinet" was extended: he presents the holders of the various portfolios to the King, and chairs the Council in the King's absence.
The cabinet of the "Prime Minister" was introduced by the Royal Decree of 25 November 1918. From that date, the title is used in official documents. Over the course of time, the King has chaired the Council less and less, so that the political role of the Prime Minister has increased, though without establishing his priority. For years, he combined this office with another portfolio.
After the second World War, the Prime Minister became the head of the Government. His post also became more complex, as he is generally the leader of a coalition, and consequently has to act as a mediator between ministers from different parties.
We will have to wait for the reform of 1970 to see the title of Prime Minister recognised in the Constitution (Art. 86bis, now Art. 99, paragraph 2). Finally, the constitutional reform of 1993 expressly relates the function of the Prime Minister to the formation or dismissal of the Government (Art. 96).