The number of people developing a drug problem behind bars has more than doubled in just five years, according to analysis by the Reform think tank.

They found that almost 15% of prisoners reported being drawn to illegal drugs after going to jail.

To carry out the study, they looked at the results of surveys of inmates carried out between 2013/14 and 2018/19.

The findings suggest the proportion of prisoners who said they had experienced this problem rose from around 6.4% to about 14.8%.

Researchers looked at a sample size of 7,097 inmates who responded to the survey in 2013/14 and 5,845 in 2018/19 to calculate the findings.

The numbers were drawn from several samples of responses and the percentages are estimates, not exact figures.

Aidan Shilson-Thomas, a researcher at Reform and author of the report which also reviewed the state of prisons, said the findings suggested the drug problem in prisons has become ‘more serious’ in the last five years.

The organisation’s review echoed warnings raised by prison inspectors of poor living conditions, overcrowding and problems with keeping experienced staff in their jobs.

Investment in equipment and staffing is required for long-term reforms, the think tank’s report warned, arguing that short prison terms were adding to problems.

Reform called on the Government to carry out an urgent review to consider ending the use of short-term prison sentences.

Mr Shilson-Thomas said prison needed to be an ‘opportunity for inmates to change their behaviour’, adding: ‘Stabilising the system means stemming the flow of drugs, reducing overcrowding, fixing the crumbling estate and improving officer retention.

‘Its long-term sustainability requires a serious conversation about how many people we lock up and for how long.

‘Failing to act will mean poorer social outcomes, more re-offending and ultimately huge costs to the taxpayer.’

A Ministry of Justice spokesman said: ‘Illicit substances pose huge challenges in our prisons which is why we are investing £100million in airport-style security – including x-ray body scanners – to stop them getting in.

‘This is part of our £2.75 billion investment to make jails safer for offenders and staff, while working closely with healthcare providers to ensure prisoners have the support they need to live drug-free upon release.’