Drug-users can be sent for treatment instead of imprisonment

Only 28% of the punishments finished last year were completed by offenders, a National Audit Office report said.

But the orders (DTTOs) can be successful for some drug abusers as an alternative to jail, auditors added.

DTTOs were launched in 1998 to try and wean drugs users who commit crimes off their habit rather than imprison them.

Edward Leigh MP, chairman of the Commons' Public Accounts Committee, said: "Much more needs to be done to ensure offenders comply with the terms of the order."

Drug tests

The orders require offenders to have two drug tests a week and initially complete between 12 and 20 hours of contact a week with probation workers and counsellors.

The order can help some offenders turn their lives around Sir John Bourn National Audit Office head Listen to one addict's success story Addicts back scheme

But completion rates varied "considerably" across the country from 71% in Dorset to just 8% in Kent, the watchdogs found.

The DTTOs cost between �5,200 and �7,600 per order, or between �25 and �37 a day compared with �100 a day to keep someone in prison, they added.

Sir John Bourn, head of the National Audit Office, said: "The order can help some offenders turn their lives around and reduce their use of drugs.

"However, the high drop-out rate and evidence from pilots of the order of a high rate of reconviction need to be addressed."

The report found that the re-conviction rate was lower for those who had completed their DTTOs, at 53%.

'Government obsession'

Harry Fletcher, assistant general secretary of probation union Napo, told BBC News Online the failure rate was artificially high because of government pressure to take candidates who were unsuitable.

"In principle linking treatment to supervision is a sound idea so long as the treatment orders are given to people who have a realistic chance of completing them.

"But staff have been pressured, because of the government's obsession with targets, to take more and more people on orders."

Home Office minister Paul Goggins said: "We intend to reduce the numbers of people in treatment who fall through the gaps."

More than 18,400 orders were imposed across England and Wales by December last year.