According to reports in The Sun, Miranda was offered a "significant" role by Steven Moffat, but BBC controller Danny Cohen prevented her from taking the role because he wants her to concentrate on her show, Miranda.
A source told The Sun:
"Miranda was thrilled to be asked to be on Doctor Who. Steven offered her a decent role and she felt, as it was on the BBC, there would be no problem. But the BBC One controller let it be known that he wants her focused on her show, which he has nabbed from BBC Two. Miranda's gutted as she feels she could do both."
What do you think about comedians taking on roles in Doctor Who? Remember Peter Kay's not-so-popular Abzorbaloff?