A search is presented for direct top squark pair production using events with at least two leptons including a same-flavour opposite-sign pair with invariant mass consistent with the [Formula: see text] boson mass, jets tagged as originating from [Formula: see text]-quarks and missing transverse momentum. The analysis is performed with proton-proton collision data at [Formula: see text] collected with the ATLAS detector at the LHC in 2012 corresponding to an integrated luminosity of 20.3 fb[Formula: see text]. No excess beyond the Standard Model expectation is observed. Interpretations of the results are provided in models based on the direct pair production of the heavier top squark state ([Formula: see text]) followed by the decay to the lighter top squark state ([Formula: see text]) via [Formula: see text], and for [Formula: see text] pair production in natural gauge-mediated supersymmetry breaking scenarios where the neutralino ([Formula: see text]) is the next-to-lightest supersymmetric particle and decays producing a [Formula: see text] boson and a gravitino ([Formula: see text]) via the [Formula: see text] process.