The results of a search for pair production of the lighter scalar partners of top quarks (˜t1) in 2.05 fb−1 of pp collisions at √s = 7 TeV using the ATLAS experiment at the LHC are reported. Scalar top quarks are searched for in events with two same flavour opposite-sign leptons (e,μ) with invariant mass consistent with the Z boson mass, large missing transverse momentum and jets in the final state. At least one of the jets is identified as originating from a b-quark. No excess over Standard Model expectations is found. The results are interpreted in the framework of R-parity conserving, gauge mediated Supersymmetry breaking ‘natural' scenarios, where the neutralino (χ 0 1) is the next-to-lightest supersymmetric particle. Scalar top quark masses up to 310 GeV are excluded for 115 GeV < m χ 0 1< 230 GeV at 95% confidence level, reaching an exclusion of mt1 < 330 GeV for m ˜χ 0 1 = 190 GeV. Scalar top quark masses below 240 GeV are excluded for all values of m ˜χ 0 1>mZ.