The discovery(1) of high-temperature superconductivity in copper oxides raised the possibility that superconductivity could be achieved at room temperature. But since 1993, when a critical temperature (T-c) of 133 K was observed in the HgBa2Ca2Cu3O8+delta (ref. 2), no further progress has been made in raising the critical temperature through material design. It has been shown, however, that the application of hydrostatic pressure can raise T-c-up to similar to 164 K in the case of HgBa2Ca2Cu3O8+delta (ref. 3). Here we show by analysing the uniaxial strain and pressure derivatives of T-c, that compressive epitaxial strain in thin films of copper oxide superconductors could in principle generate much larger increases in the critical temperature than obtained by comparable hydrostatic pressures. We demonstrate the experimental feasibility of this approach for the compound La1.9Sr0.1CuO4, where we obtain a critical temperature of 49 K in strained single-crystal thin films-roughly double the bulk value of 25 K. Furthermore, the resistive behaviour at low temperatures (but above T-c) of the strained samples changes markedly, going from insulating to metallic.