tomato fruits going brown

Hello, Im growing some heirloom tomato varieties and Ive just noticed that the fruit on the green sausages is going brown and soft at the furthest end from the stem. What have i done wrong? They are in pots outside and are fed once a week with tom fertilizer. i should probably mention that they arnt yet ripe. it dosnt seem to have affected the other varieties yet. any advice appreciated. should i remove the damaged fruit?
thanks

 

I'm new to all this myself but saw this and thought it might help.

http://www.gardenerscorner.co.uk/forum/showthread.php?t=15863

Laura x

 

Sounds like blossom end rot, to me. Caused by inconsistant watering when the fruits are filling out. No cure, but preventable by keeping the soil moist but not waterlogged. Pick off affected Toms.

 

I had problems with blossom end rot but was advised to try out Plant Magic from www.gardenwizard.net it is Calcium based and great at preventing diseases in all my plants.

Guildyear