Impossible to stop an unauthorised debit.

I am hopelessly trying to stop and reverse a debit order. that I did not authorise. it's impossible to get through to speak to anyone to help 1. the debit order does not appear in my online banking profile 2. if I try to call the call centre, they require my ID number so I can't get through because I am using a passport 3. the secure chat is telling me that I am number 1870 in the queue it seems impossible to stop this unauthorised debit. Please help.