Prisons may have released thousands of drug addicts back onto British streets with misleading "drug free" certificates, it has emerged.

Prison bosses have now been told the testing system only picks up one or two narcotics - not all.

From next week, the documents must all include disclaimers revealing the gaping flaw, making them worthless.

The revelation should cause concern to employers who may have unknowingly taken on drug-addled ex-inmates.

The shambles in the voluntary scheme was exposed by Shadow Justice Secretary Damien Green.

The senior Tory MP blasted: "What signal does it send out?"

Prisoners are tested for nine illegal drugs.

But the "significant majority" of urine samples are only tested for one or two, a leaked Prison Service document reveals.

The certificates are issued to prisoners tested voluntarily and are widely used by them on release to convince potential employers and public bodoes that they are 'drug free'.

However they are widely open to abuse. Junkie singer Pete Doherty has told how he faked a drug-free certificate on his release from jail in May 2008.

New rules issued to prison governors this month warn the only reliable way to prove convicts are drug-free is to test for all types of drugs at four-day intervals. The voluntary scheme only tests at 20 to 30 days.

A Prison Service spokesman said: "Voluntary drug test certificates just explain a prisoner has undergone a test and got a negative result."