Husnain Rashid – who also planned to poison supermarket ice cream – had his face slashed with a makeshift knife.
The attack, using a toothbrush with a blade attached to it, left the Islamist extremist with a huge gash from his right ear.
It happened on Wednesday morning on the Category A wing at Manchester’s Strangeways jail.
A source said: “There was blood all over his cell and the landing. Nobody likes him or what he did, like threatening that young royal lad and all the icecream stuff. We don’t tolerate that kind of thing in Strangeways.”