A Home Office research briefing says many regular crack users take a variety of other drugs, including heroin.

Although crack use is rare, they fear it could increase if it is made more available and could lead to recreational drug users getting hooked on heroin.

At the same time, they say dealers are becoming more sophisticated, with many users ringing through their orders by mobile phone and getting home deliveries.

Heroin

The Home Office research focuses on crack users in north west England.

Researchers interviewed 63 long-time crack users in 1995. Two years later they interviewed 50 of these users again and 29 new users.

Of the 50, 13 had come off crack, 15 were reducing their drug taking and 22 were confirmed addicts who used crack daily with other drugs, mainly heroin and methadone.

Many regular crack users also use heroin

They found that the people who had given up crack had also cut down on use of other drugs.

But those who were reducing their crack use were nearly all continuing to use other drugs, including methadone.

The crack addicts were increasingly reliant on illegal means to fund their habits, mainly prostitution, drug dealing and robbery.

Their drug bills were on average between �10,000 and �25,000 a year, although some were as high as �135,000 a year.

The combined annual drugs bill for addicts and those who were reducing crack use was around �20,000 a year and had not reduced much since 1995.

The 29 new users, who had only taken the drug for two years, tended to mix crack with other hard drugs, particularly heroin and methadone.

Stone

The researchers also interviewed new recreational users of crack who call the drug rock or stone.

They did not use heroin, but were likely to mix the drug with cannabis, amphetamines and Ecstasy.

They thought they were in control of their cocaine use. Most did not resort to crime to pay for their habit and came from stable backgrounds.

The researchers said that continued and more frequent use of crack led to psychological dependency.

They fear this could mean that recreational use of crack becomes a bridge to harder drugs like heroin.

Drug charities say the use of heroin is increasing in certain areas of the UK, particularly in cities outside London.

Image and price

The Home Office report says the main barriers to using harder drugs is image and price.

They believe crack still has an "ambivalent" image and, at �20 a rock, is seen as expensive by recreational drug users.

But they fear the image of cocaine is softening and dealers could lower the price of crack in order to expand their market.

They are also worried that crack dealing has become more sophisticated, with dealers preferring a "home delivery service" because of crackdowns on street dealing.

Many dealers now operate by mobile phone and some use very young runners to deliver to users.

Several sell both crack and heroin at the same time because of the link between the two drugs.

The report calls for more research into hard drug use in the UK. It says the current lack of information on drug trends means it is "almost impossible" to identify emerging drug problems.

They also state that the criminal justice system is failing to help hard drug users off drugs, either through imprisonment or drug treatment programmes.

History of crack

Crack is a purer, more concentrated form of cocaine. It first appeared in US cities in the 1970s and was developed as a way of testing the purity of cocaine from South America.

Researchers fear recreational use of crack could lead to harder drugs

Crack use is still fairly rare outside big cities. The drug is smoked rather than snorted and is quickly absorbed into the body.

Like cocaine, it gives a general feeling of wellbeing, sharpness and physical strength.

But side effects include feelings of panic, depression and anxiety and large doses can induce paranoia.

In rare cases, overdose can lead to heart failure and death.

Crack's effects are more intense than those of cocaine and the high usually wears off after 15 minutes.