Removal of mother as guardian

headwrecked09

I as mother automatically have guardianship but I had a solicitor letter sent to me ex to discuss costs. We have had a happy enough access arrangement for over 2 years. However I’ve now received notice that he is requesting my removal as guardian of our child. I’m stunned and feel sick at the idea of it but also how he would attempt to do this and why.
If anyone has heard of a mother being removed as guardian for reasons I can’t see I’d appreciate any experience. I have a solicitor whom I’m waiting to hear back from.