When submitting a GET form, the query string is removed from the action URL

Consider this form:

<form action="http://www.blabla.com?a=1&b=2" method="GET">
    <input type="hidden" name="c" value="3" /> 
</form>

When submitting this GET form, the parameters a and b are disappearing.
Is there a reason for that?
Is there a way of avoiding this behaviour?