Donaldson represents a "very complex challenge to the Northern Ireland Prison Service", Acheson added.

"There are always hierarchies within prisons - and there are even hierarchies within sex offenders - and the lowest of the low and the most vulnerable in terms of hostility from others are those who've been convicted of sexual violence against a minor," he said.

But he said it would also be necessary to "examine potential political or sectarian targeting" relating to Donaldson's leadership of the DUP and his service in the Ulster Defence Regiment, a former Army unit.

"The prison has a duty of care to protect him and he is uniquely vulnerable," Acheson told BBC Radio Ulster's Talkback programme.

"Maghaberry, in previous inspections, has indicated that it operates an integrated regime, where sex offenders can be safely managed in the wider prison community.

"I don't think that is remotely viable for Jeffrey Donaldson, given those offence characteristics and his background.

"So either he is kept, in effect, in conditions of solitary confinement for years and years or another solution has to be found.

"I don't think it's acceptable to keep him in conditions of solitary confinement - it may not even be legal."