Upper bound on all products of R-parity violating couplings $\lambda'$ and $\lambda''$ from proton decay

We prove that {\em any} product of R-parity violating couplings $\lambda'$ (L-violating) and $\lambda''$ (B-violating) can be strongly restricted by proton decay data. For any pair $\lambda'$ and $\lambda''$ the decay exists at least at one loop level. For squark masses below 1 TeV we find the conservative bounds $|\lambda' \cdot \lambda''| < 10^{-9}$ in absence of squark flavor mixing, and $|\lambda' \cdot \lambda''| < 10^{-11}$ when this mixing is taken into account. We study the dependence of the bounds on the flavor basis in which R-parity breaking couplings are determined.