Convicts are to receive Government help to ensure they can inject heroin safely inside jail.
Under the dramatic relaxation of drugs policy, criminals will be given taxpayer-funded disinfectant tablets to clean their syringes.
The Conservatives said the 'outrageous' decision effectively condoned drug use by inmates, who are supposed to be given help to quit Class-A substances.
Theft and robbery to pay for drug addiction is one of the main reason criminals end up in jail in the first place, they said.
Possession of injecting equipment by prisoners is illegal, but officials said they had a right to be protected from blood infections.
There will be concern the decision - to be implemented by the Ministry of Justice later this year - is the start of a Government retreat from trying to get inmates off drugs. It follows a bruising human rights defeat in which the Prison Service was ordered to pay ?750,000 to nearly 200 drug addicts who experienced withdrawal symptoms after they were forced to go 'cold turkey'.
Shadow Home Secretary David Davis said: 'This is exactly the wrong approach and it is outrageous the Government is content to allow it. Drugs are a scourge on society which fuel much other crime. Prisons should be places of rehabilitation where prisoners are got off drugs, not left on them.
'That is why we would strengthen residential and prison drug rehabilitation places to get people off drugs and start reducing crime.'
It has also emerged that homosexual inmates are to be given access to gay pornography to ensure that they do not feel discriminated against.
The Prison Service has ruled they should be allowed any type of explicit 'top shelf' material which can be bought in a high street newsagent.
The decision was taken after gay inmates complained they were receiving 'unequal treatment', because heterosexual prisoners are already allowed pornographic magazines.
Guidance given to governors says the magazines 'should be allowed into the establishment and that gay/bisexual prisoners should not be discriminated against'.
Matthew Elliott, chief executive of the TaxPayers' Alliance, said: 'Prisoners should not be allowed any porn - gay or straight - when they're serving time, and neither should they be provided with drugs paraphernalia.
'The Human Rights Act has obliterated common sense from our criminal and legal system. The idea that families and pensioners are paying higher taxes to fund drug-fuelled, porn sessions for crooks will offend all right-minded, decent people.'
But Karen Munro, of the Prison Service's briefing and casework unit, said: 'The possession of injecting equipment is strictly prohibited in prisons and liable for punishment. Any equipment found is confiscated.
'It is, however, recognised that blood borne viruses are a serious public health problem, and are more prevalent in the prison environment than in the community.
'The Ministry of Justice and the Department of Health are therefore working to introduce disinfectant tablets in prisons.
'These are a generic cleaning item that can be used to clean shaving equipment, cutlery, crockery, cell toilets and toothbrushes and may be used to sterilise illicitly held needles which may be used for drug-injecting or tattooing.'