  The results of a search for gluino and squark pair production with the pairs
decaying via the lightest charginos into a final state consisting of two $W$
bosons, the lightest neutralinos ($\tilde\chi^0_1$), and quarks, are presented.
The signal is characterised by the presence of a single charged lepton
($e^{\pm}$ or $\mu^{\pm}$) from a $W$ boson decay, jets, and missing transverse
momentum. The analysis is performed using 139 fb$^{-1}$ of proton-proton
collision data taken at a centre-of-mass energy $\sqrt{s}=13$ TeV delivered by
the Large Hadron Collider and recorded by the ATLAS experiment. No
statistically significant excess of events above the Standard Model expectation
is found. Limits are set on the direct production of squarks and gluinos in
simplified models. Masses of gluino (squark) up to 2.2 TeV (1.4 TeV) are
excluded at 95% confidence level for a light $\tilde\chi^0_1$.
