Exploiting several ℓ p -factorization results for strictly singular operators, we study the strict singularity of the multiplication operator L A R B : T → AT B on L(X) for various Banach spaces X.2010 Mathematics Subject Classification. 47B47,46B28. Key words and phrases. Multiplication operator, strictly singular operator, L p space. The second-named author acknowledges financial support from the Spanish Ministry of Economy and Competitiveness through grants MTM2016-75196-P, MTM2016-76808-P, and the "Severo Ochoa Programme for Centres of Excellence in R&D" (SEV-2015-0554). Support of an LMS Research in Pairs grant is also gratefully acknowledged. We thank Prof. W. B. Johnson for helpful discussions concerning this paper. We are grateful to the referee for carefully reading the paper and pointing out an oversight in a first version of the proof of Theorem 5.5.