Self assements and LPAs

I have called HMRC National advice line to ask how I submit Personal tax returns for my mother who has severe dementia and I explained I have lasting power of attorney in place. My mother doesnt have a driving licence or passport and is incapable of signing her name or givng consent. Despite explaining that she cannot sign her name I was advised that I cannot take over submitting her tax returns without her signing an agent authorisation form. This surely cannot be correct?

Hi,
The lasting power of attorney will need to be noted on your mothers record before you can act on her behalf.
As access to your mothers records will be needed you will need to contact HMRC direct:
Self Assessment: general enquiries
Thank you.