Cregan, 30, is now in the same secure mental hospital as Moors Murderer Ian Brady, after being moved from Strangeways jail in Manchester.
The father of one, who shot Moray-born Pc Fiona Bone, was put in the hospital wing at the jail before his transfer yesterday after refusing food.
It had been thought Cregan, serving four life sentences, would be moved to HMP Full Sutton near York, after his trial in June. But he remained at HMP Manchester.
Cregan has been kept segregated from other prisoners for fear of reprisals.
He was jailed for life at Preston Crown Court for the murder of two unarmed policewomen and a father and son. He was told he would spend the rest of his life behind bars. But a police source said his behaviour meant he was to be held at Ashworth.
"His head has gone," the source said. "He can't take it any more, he's not cocky any more, what a difference in attitude. He's not eaten for four weeks, he's gone in the head."
Ashworth Hospital, near Maghull on Merseyside, houses prisoners deemed to be criminally insane.
Cregan lured Pcs Nicola Hughes, 23, and Fiona Bone, 32, to their deaths in a horrifying gun and grenade attack on September 18 last year.
He had gone on the run days before, killing David Short, 46, last August after gunning down his son Mark, 23, at the Cotton Tree Pub in Droylsden three months earlier.