Do you need to warm up for stretching?

Okay so i have seen many different types of answers, do you need to usually warm up before stretching? everytime i have stretched, i usually go for a 20 min power walk or 5-10 min jog etc?

What happens if you don't warm up? is stretching not effective? will it not increase your flexibility at all? or is it more likely to damage muscles?

thanks for the replies.