A method of installing a formwork support system, comprising the steps of: a. arranging a first pair of support props having head members in an upright position, b. connecting opposite ends of a first longitudinal beam to the head members of the first pair of support props, c. arranging a second longitudinal beam in an interim mounting position by releasably connecting a first end of the second longitudinal beam to the first longitudinal beam, the second longitudinal beam freely projecting downwardly from the first end towards the second end, a stop element at the first end of the second longitudinal beam bearing against the first longitudinal beam, a hook element of the second longitudinal beam, in the interim mounting position, engaging a slot of the first longitudinal beam, d. arranging the second longitudinal beam in a horizontal support position by lifting the second end of the second longitudinal beam.