Relativistic positron beams are required for fundamental research in nonlinear strong field QED, plasma physics, and laboratory astrophysics. Positrons are difficult to create and manipulate due to their short lifetime, and their energy gain is limited by the accelerator size in conventional facilities. Alternative compact accelerator concepts in plasmas are becoming more and more mature for electrons, but positron generation and acceleration remains an outstanding challenge. Here we propose a new setup where we can generate, inject and accelerate them in a single stage during the propagation of an intense laser in a plasma channel. The positrons are created from a laserelectron collision at 90 degrees, where the injection and guiding are made possible by an 800 nC electron beam loading which reverses the sign of the background electrostatic field. We obtain a 20 fC positron beam, with GeV-level central energy within 0.5 mm of plasma.