Drugs are "readily available" at the UK's biggest prison, a report has found.
The Independent Monitoring Board's (IMB) first report into the Berwyn super-prison in Wrexham found 20% of prisoners did not feel safe, with drugs a common reason given for violence.
It comes days after staff raised concerns about being assaulted.
The Ministry of Justice acknowledged prisoner drug use is a "serious issue".
The IMB said while prisoners were treated with respect and dignity there were "very real concerns at the ingress of illicit drugs, which appear to be readily available within the establishment," at the £250m HMP Berwyn, which opened in February 2017.
The report added: "A local violence reduction survey was conducted in January 2018. In response to a series of questions 81% of the men who completed the survey said that they felt safe at Berwyn.
"However, that indicates almost 20% who did not. Men identified drugs and resultant debt as being the most common reason for incidents of violence."
It also said when men have become "so affected" by drugs and debt issues, they feel their only option is "to take refuge" in the prison's care and separation unit.
The lack of the promised work places and completion of the workshops has been an "ongoing concern".
Prisons Minister Rory Stewart said the report praised the prison for doing a "great job at reforming offenders", but the high rates of drug use by prisoners remained a serious issue.
"We're already taking action to combat this including training sniffer dogs, rolling out body scanners and investing £14m a year to disrupt organised crime in prison," he said.
However, the report also praised the running of the new prison, saying men were "treated fairly and with decency", "with every effort made to accommodate those who find custody, and the move towards rehabilitation, particularly challenging."
It said the emphasis on humane treatment "comes right from the top" and also highlights the "enormous amount of work carried out to prepare men for release and resettlement".
It reads: "The setting up and operation of Berwyn is still work in progress, with the establishment working towards finding and maintaining the right balance between rehabilitation and security, between rewards and sanctions and between freedom and control, in order to make it an enduringly safe and rehabilitative prison.
"The challenge of building an establishment the size and complexity of Berwyn 'from the drawing board' cannot be over-emphasised and the fact that it is operational, with many examples of good and innovative practice is a considerable achievement."