The coupling of the electroweak gauge bosons of the Standard Model (SM) to leptons is flavour universal. Extensions of the SM do not necessarily have this property. Rare decays of heavy flavour are suppressed in the SM and new particles may give sizeable contributions to these processes, therefore, their precise study allows for sensitive tests of lepton flavour universality. Of particular interest are rare $b \to s \ell \ell$ decays that are well accessible at the LHCb experiment. Recent results from LHCb on lepton flavour universality in rare $b \to s \ell \ell$ decays are discussed.