  A measurement of the production of $W$ bosons with opposite electric charges
in association with two jets is presented based on 140 fb$^{-1}$ of data
collected by the ATLAS detector in proton-proton collisions at $\sqrt{s}=13$
TeV. The analysis is sensitive to the scattering of $W$ bosons, which is of
particular interest in the ATLAS physics programme as it can be used to probe
the electroweak symmetry breaking mechanism of the Standard Model. This signal
is observed with a significance of 7.1 standard deviations above the background
expectation, while 6.2 standard deviations were expected. The measured
cross-section is determined in a signal-enriched fiducial volume and is found
to be $2.7\pm0.5$ fb, which is consistent with the theoretical prediction of
$2.20^{+0.14}_{-0.13}$ fb.
