Drugs are still getting into Hull Prison even though a body scanner has been introduced, according to prison officers.
The Ministry of Justice says the new scanners, also installed at Humber, Leeds and Lindholme jails among others, have already caught a number of smugglers.
The machines can also detect drugs on clothes and in prisoners' post.
But despite this, drugs are still making their way to prisoners, officers say.
Rob Nicholson, from the Prison Officers' Association, said: "We're up against new drugs. You've also got people who are attempting to bring stuff in.
"So while you're battling to try and counteract everything, there's always somebody adapting new ways of getting it in. They're still getting [the drugs] into prison."
A Prison Service spokesperson said: “We are spending an extra £2.75bn to transform our jails with tough new security measures - to improve safety and cut off the flow of drugs.”