Prisoners are getting high on boxer shorts and Y-fronts soaked in zombie- drug Spice.
The mind-bending substance is being washed into underwear and sent in by pals.
Lags rip off bits of the garments and smoke them. The ruse is being used after jail chiefs rumbled letters and cards were being soaked in the liquid form of the drug.
A source said: “Inmates will never give up on finding new ways to get their fix.
“These drug undies are big business in the smuggling game.”
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The first prison to cotton on to the contraband is HMP Rochester in Kent, home to 700 men.
Jail chiefs now insist every pair of pants and socks sent in must be checked by a Rapiscan Itemiser drug-detecting machine.
But it has triggered a shortage of pants among lags, who can wear their own clothes.
Hundreds have moaned about personal property being withheld.
Spice, officially referred to as a New Psychoactive Substance, or NPS, is one of the most widespread drugs in UK prisons.
It is nicknamed “bird killer” because it kills time while “doing bird”.
The drug undie issue was raised by HM Inspector of Prisons after a visit to Category C jail Rochester, where one in four inmates said it was easy to get drugs.
The Prison Service said: “We are spending £100million to stop drugs getting in, including the use of X-ray body scanners.”
