A low-pressure mercury vapor discharge lamp has a light-transmitting discharge vessel (10) enclosing, in a gastight manner, a discharge space (11) provided with a filling of mercury and a rare gas. The discharge vessel (10) comprises discharge means for maintaining a discharge in the discharge space (13). At least a part of an inner wall (12) of the discharge vessel (10) is provided with a protective layer (16). The discharge vessel (10) is provided with a luminescent layer (17) comprising a luminescent material. The luminescent layer (17) further comprises inorganic softening particles (27) with a softening point above 450.degree. C. The size of the softening particles (27) is in the range from 0.01 to 10 gm. Preferably, the softening particles (27) are selected from the group formed by strontium borate, barium borate, yttrium borate, yttrium-strontium borate and calcium pyrophosphate.