An experimental study of the Rayleigh-Benard convection in a yield stress fluid (carbopol (R) 980) uniformly heated from below in a rectangular cavity with high aspect ratio is presented. By combined integral measurements of the temperature difference between two parallel plates and the local flow velocity within a wide range of heating powers P two distinct regimes are observed. For heating powers smaller then a critical value P-c a purely conductive regime is observed. A gradual increase of the heating power beyond this onset reveals a convective regime manifested through a nonlinear dependence of the temperature difference between plates on the heating power. Simultaneously with this, local measurements of the flow fields reveal a nonlinear increase of the roll pattern amplitude. Regardless the concentration of Carbopol (R) and in spite of a significant shear thinning behaviour, the Rayleigh-Benard convection in the Carbopol (R) gel is found to emerge as an imperfect bifurcation that can be correctly modelled by the Landau theory of phase transitions. A critical slowing down phenomenon is observed corresponding to the onset of convection. The scaling laws of the convective onset P-c and of the corresponding temperature difference Delta T-c with the relevant material properties are discussed. The onset of the instability can be described in terms of a critical yield number rather than in terms of a critical Rayleigh number. The paper closes with a comparison of our findings with existing previous works. (C) 2013 Elsevier B.V. All rights reserved.