A non-intrusive method of measuring electric fields in plasmas is presented. It is based on the use of an atomic hydrogen beam prepared in the metastable fine structure quantum state 2s1/2. Interaction with the field that is to be measured causes Stark mixing with the closely lying 2p1/2 (Lamb-shift), whose spontaneous decay rate is much higher than that of 2s1/2. As a result, the total transition rate to the ground state (and consequently the intensity of the Lyman-α line at 121.6 nm) is proportional to the square of the electric field amplitude, thus providing a very sensitive method to measure it.