Johann Hari: The hidden scandal: There are thousands detained without trial for more than 90 days

More than 12,000 people are banged up like this every year, only to be found not guilty of any crime

The idea of detaining people without trial for 90 days did not die in the voting lobbies of the House of Commons last Wednesday. It is already a reality in Britain's prisons, and it is getting worse. While our MPs congratulate themselves on striking a blow for liberty, in this country tonight there are more than 6,000 people who have been jailed for more than 90 days, even though they have been convicted of nothing at all.

They have not been accused of the headline-grabbing crimes of jihadism, but mostly of unsexy, non-violent offences: theft, drug-dealing, breaching Asbos. They are Britain's remand prisoners - the unlucky thousands who are denied bail and left to fester in our Victorian jails while they wait for trial.

Sitting in the dank bowels of Wormwood Scrubs, Sanjay - a 32-year-old Asian accused of selling ecstasy, which he claims he had solely for personal use - tells me what it is like to live through this. "It eats you up, sitting in a cell knowing you're innocent. The cell door bangs shut on a Friday night and it's not really open again until Monday morning, and you sit there thinking: 'Fuck me, I'm not convicted - why am I in here?'

"I've lost my wife, I'm losing my house - and, if I'm found not guilty, I'll get no compensation for every day I'm spending in here," he continues. Sanjay has been remanded because, when he was 14 and living in care, he skipped bail by running away to London. This makes him a "flight risk". He doesn't know when his trial will come up, so he feels he is locked up here indefinitely. "If you're sentenced, you've got a date to work towards. When you're on remand, you could be walking out next month or in 10 years. You've got nothing, no picture of how your life's going to be. There's a bloke on my wing who has been on remand for nearly two years. Am I going to be here that long when I haven't done anything?"

More than 12,000 British people are banged up like this every year, only to be found not guilty of any crime when their trial finally arrives. As we walked through the prison, past the metal hatches, Dave Pemberton, the Scrubs' violence reduction co-ordinator, outlined the consequences of this system: "Remand prisoners are particularly vulnerable to self-harm and suicide. There's a big risk they will commit suicide before they even get to trial."

They are notorious for having "Shredded Wheat arms" - not an image I want to dwell on - and Pemberton says some prisoners are so distressed at being jailed without even a conviction that it is almost impossible to stop them "cutting up". "The stress of it can be terrible. We've had remand prisoners who self-harmed even when everything was taken away. We take away the blades and the cutlery, so one guy self-harmed with a paper plate, using it to cut into his throat. So we took that away, and he began self-harming with his food. He sharpened a chicken-bone and cut into his wrists." Of course, sometimes remand is necessary. The Bail Act of 1976 lays out the extreme circumstances in which it is sensible to lock somebody up prior to trial: if there is a serious risk they could disappear, if they have been convicted before of similar offences and might do it again, or if they are interfering with witnesses.

In one cell, I met Anthony, a strapping, strutting 42-year-old accused of attempted murder, incredulous that he has been remanded just for "talking to the witnesses a bit". Nobody would say he should be walking the streets.

But the reasons for imprisoning people without trial have expanded over the past decade, far beyond the terms of the Bail Act. Frances Crook of the Howard League for Penal Reform told me: "Remand is now being used inappropriately to deal with all sorts of problems. Often, when there are no psychiatric beds, somebody will be remanded just so their psychiatrist can actually know where they are." I saw evidence of this: I spoke to one unconvicted prisoner who was so mentally disturbed he did not know where he was, asked if I knew his father, and then assumed I was a judge with the power to release him.

"And anybody who is registered as No Fixed Abode - without an address - will automatically be remanded as a flight risk now," Crook continued. "That's effectively jailing them for being homeless. It brings the homelessness statistics down. But the biggest scandal is that magistrates are increasingly sending prisoners into remand because they believe they are guilty but they know there isn't enough evidence to convict, so they want to give them some kind of punishment before they're acquitted. It's outrageous."

In theory, remand prisoners are supposed to be treated better than convicts on the grounds that they are innocent until proven guilty. Until the early 1990s, this happened, to a degree. They were held in separate, much more open wings, and their families could visit as much as they liked, bringing in food parcels. The Whitemoor breakout - when several remanded IRA prisoners abused this system to escape - ended all that.

Today, remand prisoners are actually treated worse: they live mixed up with convicted prisoners, often sharing the same cell and regimen, but they have far less access to rehab, education or work. Their only "perks" are that they are allowed to wear their own clothes and can top up their 50p-a-day budget with private savings (if they have any). Sanjay said: "I'd be better off if I actually had done it and had been convicted than I am now. As it is, I just lie on my bed all day going mental."

After eight years of cramming our jails with ever-more prisoners, the Labour government has finally acknowledged the remand scandal. Last month, Charles Clarke said he would like to see better treatment of remand prisoners - and specifically a return to separate remand wings. But unless this is backed with hard cash and a commitment to cut the massive number of prisoners in our jails, it is just fantasy.

Sitting high above the prison in his office, Scrubs' governor, Luke Sergent, told me: "In practical terms, it's very difficult. We're operating at capacity here. We're full. If Clarke wants to deal with this, they've got to put fewer people in prison. A lot of these remand prisoners could be tagged." And a lot should be in psychiatric hospitals, rather than jail, I say. "That's true," he nods.

Sanjay slumps back in his chair on the 98th day of his imprisonment without trial. "It's a scary place, Wormwood Scrubs. Last night I couldn't sleep because there was somebody smashing up his cell next to me. What am I doing in this place? Her Majesty's Pleasure ... I'll tell you what, if this is what gives Her Majesty pleasure, she must be one sick bitch." So when are all the MPs who (rightly) made a fuss about three or four terrorism suspects going to start speaking out for tens of thousands of remand prisoners?

j.hari@independent.co.uk