Almost 2,500 prisoners play the lottery from jail.
They use smuggled mobiles to go online rather than buying tickets.
The revelation raises the prospect of another fiend hitting the jackpot – like infamous lotto rapist Iorworth Hoare.
Unlike today’s tech-savvy lags, evil Hoare bought a ticket while on day release and scooped £7.2million in 2004.
A source said: “The lottery is a big thing in jail. People never give up hope of striking it rich.”
The figures emerged in the first ever report on gambling behind bars.
Prison think-tank The Forward Trust said around 3% of the UK’s 80,000-plus inmates play the lottery.
They are among an estimated 17,500 who place bets from jail.
Others do the football pools or use mobile apps and online bookies to have a flutter on horses, dogs and sport – with some landing in financial peril.
The trust says addicted cons need more help to break the habit.
Boss Mike Trace said: “This ground-breaking survey confirms the prevalence of the problem as well as its contribution to crime.
“The findings clearly point to the urgent need to develop and test a range of programmes to address the problem.”
Inmates can place bets using their £10 per week jail wages or friends and relatives on the outside put money in online betting accounts for them.
Smartphones – illegal in prisons – can cost up to £1,000 inside, even for an old one, with SIM cards going for around £200-a-pop. Phones are also hired out for £20-£100 a day.
Gamblers who recognise they have a problem can call support services for free on prison phones.
The Prison Service said: “We are spending £100million on prison security, including x-ray body scanners, signal-blocking technology and dogs trained to sniff out mobiles.”
