Staff at Wrexham's £250m super-prison have claimed it is unsafe, less than 12 months after it opened.
The Prison Officers Association (POA) told BBC Wales' Newyddion 9 programme a "culture of fear" existed among staff at HMP Berwyn.
The group's chairman, Mark Fairhurst, said "control, order and discipline" needed to be instilled.
A prison service spokesman said HMP Berwyn had performed well since it had opened.
Mr Fairhurst said security was being compromised at the prison as officers had been told not to intervene straight away if they witnessed a package being passed between visitors and inmates.
He said officers had been told to "allow that visit to continue and then to search the prisoner once the visit is complete, by which time of course the substance will be secreted or passed on".
"It might not be drugs that's getting passed on during visits, it could be something more serious."
Mr Fairhurst also alleged occasions where dog handlers had being stood down because they did not want visitors being turned away, managers allowing inmates to stay out of cells during an alarm and evidence going missing when inappropriate behaviour by prisoners is reported.
He said he had raised his concerns "at the very top of the tree" but "nothing has changed".
Governors at HMP Berwyn had aimed to introduce a new prison culture, prioritising values such as kindness, respect, honesty and rehabilitation.
But a former staff member said she resigned after what she described as "bullying" and "being managed out" by staff above her.
"Because I challenged some behaviours, some managers' behaviour, that was not acceptable to them," she said, adding it had left her feeling "very upset and anxious".
"I believe if I was still there, it would have destroyed me. It would have destroyed my emotional, physical and mental wellbeing," she added.
BBC Wales has also seen a resignation letter from an experienced prison officer.
It claimed there was a "complete disregard for the safety of staff" at HMP Berwyn, and "staff have no confidence to challenge men".
It goes on to allege an occasion when managers "completely undermined" the officers "and gave the men what they wanted".
The former officer said they were told to "go away in front of the other men" which was "degrading and embarrassing".
"One of the men even stated 'that's you told', so this reinforced the men's view that us as officers aren't of any value at Berwyn," the officer said.
"This is the lowest I have ever been in my whole life.
"A lot of staff are feeling the exact same way... but will not speak up and instead are suffering in silence," the officer said.
Mr Fairhurst said according to staff he had spoken to, HMP Berwyn "is an unsafe prison".
He said: "We all want Berwyn to be a success... but let's be honest, the fundamentals and the basics include security, control, order and discipline. If you haven't got that foundation everything else will fail.
"The culture of fear is surrounding the lack of confidence for prisoners who display threatening, violent or anti-social behaviour... that has to cease."
He added: "You have to have staff supported and prisoners buying into the idea.
"But at the moment they are struggling and it will escalate if they don't tackle that situation."
A prison service spokesman said HMP Berwyn was performing well but "we know more must be done to improve safety across the estate" which included the introduction of body-worn cameras, extra CCTV, and a £100m investment to recruit 2,500 extra officers across the whole service.
"The prison also meets regularly with the POA to understand the concerns of their members, and to feedback on any relevant progress made," the spokesman added.