Ampersands reformatting to # 038; and breaking links

I am using Woocommerce with affiliate links and the external urls are in the format

http://www.awin1.com/pclick.php?p=3626919167&a=112638&m=6418

The ampersands are being translated into # 038; (http://www.awin1.com/pclick.php?p=3626919167&a=112638&m=6418) which stops the link from working.

How can I stop this happening? In an ideal world I want none of this formatting applying to hyperlinks.