Lily tells me that the moment they began her screening interview, they took away her bag – and with it her phone. Then she was told that she would be taken to a detention facility. “It was nine o’clock at night. Then they said, ‘The van is here for you’ . . . I always see those kinds of vans on TV when they are picking up criminals to go to the police, or to go to prison. When I saw myself in that van, that’s when I started thinking, ‘Wow, am I going to prison?’” It took her a month to contact her family and let them know where she was.

Stop a minute. What’s your gut feeling about this scenario? Does it sound like one of those nightmarish news reports in which a western journalist is shunted off to a holding cell by some despotic regime? Because it’s not. It is something that happened in Britain in the 21st century.

I’ve deliberately left the details vague – and given Lily a pseudonym that sounds like it belongs to a posh, white woman – because I feel quite strongly that if anyone treated white first-worlders the way our immigration system treats people from the developing world, there would be outrage. As it is, there is a shrug; or, worse, a silent agreement that this is the only way to “keep our country safe”. If the only way to keep our country safe is to treat human beings like treacherous cattle, count me out.

“Lily” is the name I’ve given to a woman who fled Gambia, where she was a reluctant practitioner of female genital mutilation. She sought asylum in Britain three years after arriving here in 2009 because a friend told her it was the right thing to do; she had no idea when she turned up at the assessment centre that staff had every right to detain her immediately and send her to Yarl’s Wood. And contrary to the stereotype, she didn’t choose Britain because it was seen as a soft touch. “I was looking for any little chance I can get to go out from Gambia,” she tells me over the phone. “Anywhere you go to in Africa, it’s easy for someone to take a bus and come and take you. So I had the chance to come to Britain. That’s why I seek my asylum here.”

Lily spent five months in Yarl’s Wood, the Serco-run women’s detention centre in Bedfordshire. The company is careful not to call it a prison, although a former inmate told a parliamentary committee there was only one difference between the two: “In prison, you count your days down, but in detention you count your days up.” In other words, in prison you at least have some idea when you’re going to get out.

There are 13 immigration detention centres in Britain but only the name of Yarl’s Wood really resonates. That is partly down to the work of Natasha Walter and her charity Women for Refugee Women, which the NS is supporting this Christmas. (Several years ago, this magazine successfully campaigned for an end to the detention of children in the immigration system.) It is also because it is uniquely alarming to have nearly 400 stateless, powerless women – the majority of whom say they are previous victims of sexual violence – held at a facility with so many male staff. At the parliamentary committee, one detainee described the problem with suicide watch: “I can tell you anybody who is [on] suicide watch has sexual harassment in Yarl’s Wood, because those male guards, they sit in there watching you at night, sleeping and being naked. You can hear them talking [about] it.” (Serco’s website says: “Decency and respect is at the centre of Serco’s agenda in looking after residents.”)

Meltem Avcil, a 21-year-old from a Turkish Kurd family who was held at Yarl’s Wood for three months in 2007, echoes that sentiment. After being taken there as a 13-year-old, she now campaigns for the closure of the centre because: “I saw what my mother went through, I saw what the women went through, I saw how guards were looking at women.” She says it is very difficult to interest people in the plight of refugees. “Negativity around asylum-seekers still exists. So it’s very hard to break the barrier and say, ‘No, you’ve been taught wrong all this time – I am an asylum-seeker, you can hear from me.’”

Avcil remembers arriving at the centre in late August; she had spent six years before that in Britain and had no idea that she was not legally entitled to the life she had led. “We arrived in a van and I was sleeping on my mum’s lap,” she tells me. “When we arrived, we were strictly searched. We couldn’t take in any glass, any technology; we just couldn’t take in anything. And then we went through eight metal doors to get to the unit.” She describes Yarl’s Wood as “a B-class prison for innocent people”.

Natasha Walter points out that such treatment is unnecessary. “There already exist good alternatives to detention. People who claim asylum have to stay in touch with the authorities through regular reporting. Detention makes the asylum process less efficient, more expensive and much more traumatic for the individuals going through it.” Avcil agrees: “The taxpayer pays £164 [per detainee] a night for Yarl’s Wood to be run and people simply do not know what they are paying for. People should just go and see that place and what they pay for themselves.”

Even leaving the centre can be disorientating and traumatic. Lily says that after five months in detention she struggled with simple tasks such as shopping – she had forgotten you could choose your items before going to the till – and crossing the road.

I ask Avcil what it will be like for the women who are held at Yarl’s Wood over Christmas. “They struggle a lot in that place to save their lives, so that they really don’t care what day of the month it is or which festive season it is,” she says. “It’s not a place to enjoy yourself or lie to yourself.”

Find out more at refugeewomen.com, by following the hashtag: #setherfree, or by supporting the New Statesman website’s Christmas campaign