Summary: A graph is said to be interval colourable if it admits a proper edge-colouring using palette \(\mathbb{N}\) in which the set of colours of edges that are incident to each vertex is an interval. The interval colouring thickness of a graph \(G\) is the minimum \(k\) such that \(G\) can be edge-decomposed into \(k\) interval colourable graphs. We show that \(\theta (n)\), the maximum interval colouring thickness of an \(n\)-vertex graph, satisfies \(\theta (n) = \varOmega (\log (n)/\log \log (n))\) and \(\theta (n) \leqslant n^{5/6+o(1)}\), which improves on the trivial lower bound and the upper bound given by M. Axenovich and M. Zheng [J. Graph Theory 104, No. 4, 757–768 (2023; Zbl 1541.05053)]. As a corollary, we answer a question of A. S. Asratian et al. [Discrete Appl. Math. 335, 25–35 (2023; Zbl 1514.05127)] and disprove a conjecture of M. Borowiecka-Olszewska et al. [Result. Math. 76, No. 4, Paper No. 200, 21 p. (2021; Zbl 1477.05073)]. We also confirm a conjecture of the first author that any interval colouring of an \(n\)-vertex planar graph uses at most \(3n/2-2\) colours.