One of Britain's largest prisons stopped using X-ray body scanners to detect contraband for "unclear legal reasons", a report has revealed.
HMP Wandsworth was questioned by inspectors as to why the "effective" technology had been removed.
Inspectors said psychoactive substances and cannabis were "too accessible" at the south-west London prison, which is the most-overcrowded jail in England.
Existing security technology was not used effectively, the assessment found.
"For example, the X-ray scanner in reception was no longer used and the CCTV cameras in visits were not monitored as a result, we were told, of a lack of staff," it said.
The X-ray scanner had been used previously and was an effective way of identifying contraband, HM Inspectorate of Prisons (HMIP) said.
Inspectors were also told inmates preferred the scan, which was more dignified than strip-searching.
"However, the scanner had fallen into disuse; we were told this was for legal reasons but we remained unclear about the reasons, given that similar scanners were in use in other establishments," the report said.
Inspectors previously highlighted encouraging results from the use of body-scanning technology at HMP Belmarsh, saying it had led to the discovery of mobile phones, drugs and weapons that would not have been detected during a strip search.
The report on HMP Wandsworth, based on inspection visits in February and March, said in the previous six months, searches recovered 277 mobile phones, 65 weapons and 153 drug packages.
In other findings, the watchdog said Wandsworth remained the most overcrowded prison in England and Wales, with most of the 1,428 men being held at the time sharing a cell built for one.
Chief Inspector of Prisons Peter Clarke said: "In essence, there were too many prisoners, many with drug-related or mental health issues, and with not enough to do."
Calling for "cultural change", he noted that not all staff carried anti-ligature knives despite six self-inflicted deaths since the jail was last inspected in 2015.
Prisons minister Rory Stewart acknowledged it was a "disappointing" report.
He said the Government was putting an extra £16m into improving facilities at 11 prisons, including Wandsworth.
A Prison Service spokesperson said:"Wandsworth took a very cautious approach to new enhanced EU regulations on body scanners.
"Their use continued as normal across other prisons with scanners and we have now advised Wandsworth how to use the scanners effectively and in line with the EU regulations."