A measurement of the production cross-section for top quark pairs (tt¯) in pp collisions at s=7TeV is presented using data recorded with the ATLAS detector at the Large Hadron Collider. Events are selected in the single lepton topology by requiring an electron or muon, large missing transverse momentum and at least three jets. With a data sample of 35 pb−1, two different multivariate methods, one of which uses b-quark jet identification while the other does not, use kinematic variables to obtain cross-section measurements of σtt¯=187±11(stat.)−17+18(syst.)±6(lumi.)pb and σtt¯=173±17(stat.)−16+18(syst.)±6(lumi.)pb respectively. The two measurements are in agreement with each other and with QCD calculations. The first measurement has a better a priori sensitivity and constitutes the main result of this Letter.