How do I stop CodeIgniter adding semicolons ; to data sent via POST that contains ampersand &?

For example it is converting "a=1&b=2&c=3" into "a=1&b;=2&c;=3". From looking on the forums it seems to be XSS filtering, which I don't want to disable site-wide only for 1 controller, so I tried the code below but it's still doing it:

$this->config->set_item('global_xss_filtering',false);