Legislation that covers the PIP invitation

What is the specific legislation which covers the so called 'PIP invitation' and gives DWP the right to subsequently cancel a lifetime award of DLA if the 'invitation' isn't accepted.

Surely the DWP is placing individuals under duress in this regard, given an individual is 'invited' to claim for PIP, but will effectively be subject to a punishment if they don't take up the offer.

Is it possible (over the phone at PIP 'invitation' acceptance) to record that a claim for PIP is being made under duress (given the above scenario) - and would that provide a potentially valid legal marker in time, to which an individual could return back to later if need be.

Do you know which piece of legislation covers the time period of sending back the pip2 form (and all other matters to do with the submission of the pip2 form). I'm sure I've come across some quite detailed information on it, in the actual legislation, previously, but have not been able to locate again.

Thanks, though which section are you seeing it in, as just looked through again and couldn't locate anything. I've also checked through: The Social Security (PersonalIndependence Payment) Regulations 2013.