Many people may take their birth certificate, or similar official papers, for granted - hidden in a drawer and rarely seeing the light of day - but for those without one, it can lead to a shadow life or an uncertain existence.

This is thought to affect millions of people worldwide, described as stateless, and 25-year-old South African Arnold Ncube is one of them.

Because he has no state-issued documents, washing cars in the backstreets of Thembisa township near Johannesburg is one of the few ways he can make a living.

He was born in Johannesburg, and his father is South African, qualifying him for citizenship here. But when he tried to register for secondary school, he realised he didn't have a birth certificate.

Having been abandoned by his parents - his dad left before he was born and his mum when he was 14 - he could not prove his status.

"It's a painful thing," he says. "You're basically invisible. You don't exist. It's like you're living in the shadows. You don't have a bank account, you can't apply for a decent job that you can earn a living with."

He adds that he tries to stay positive, but it's been challenging.

"When I see my peers, they are done with school now. Whereas I couldn't study further. It's a lot. Depression was once my friend."

Arnold is one of at least an estimated 10,000 stateless people living in South Africa who, despite being born here, are struggling to prove their nationality and access public services.

There are no official statistics available on stateless people here because they tend to slip through the cracks. So the figures are based on estimates from organisations such as the UN's refugee agency, the UNHCR, and civil rights organisations.

With no citizenship, stateless people cannot get documents and struggle to get access to basic necessities, including education and healthcare.

Many factors, including administrative barriers and poor record-keeping, cause statelessness. As a result, the real number of stateless people is hard to gauge in many parts of the world.

Human rights lawyer and advocate Christy Chitengu used to be stateless herself.

She only got South African citizenship three years ago with the help of the organisation Lawyers for Human Rights who worked on her case for free.

"I found out I was stateless at the age of 17. My high school principal called me into her office and told me that she didn't have any documents for me and that she didn't understand how I'd gotten into the school," she tells the BBC near her home in northern Johannesburg.

"I was born in Johannesburg to two foreign parents [both from Zimbabwe] and at my birth I was given a handwritten South African birth certificate."

But officials in South Africa need a printed certificate.

Christy says that when she found out she was stateless, she looked into taking her parents' nationality, but it was too late.

"I couldn't claim my Zimbabwean citizenship because by that time I was 16 and they wouldn't allow me to do a late birth registration. Also, I would have had to leave South Africa to get a Zimbabwean passport physically, and without any documents to leave the country, I wouldn't be allowed back in."

South Africa has a large number of undocumented migrants, and the authorities and local vigilante groups have been trying to crack down on irregular migration for years.

When asked whether giving stateless children citizenship could be seen as a reward to undocumented migrants who give birth in South Africa, Christy disagrees.

"I think citizenship is not a reward. It's an entitlement for someone to live a dignified life and to be seen as a human being. I think if we look at it through that lens, we realise that there's nothing that we lose by recognising a child who would otherwise not be able to go to primary school or receive healthcare."

On several occasions, the BBC contacted the home affairs department, which handles immigration issues in South Africa, to find out how it is tackling the issue of statelessness but got no reply.

Statelessness is not just an issue here, it is a huge global problem.

There's an estimated 4.5 million stateless people around the world. Some say the figure could even be as high as 15 million.

Experts believe tackling the problem requires policy changes, including allowing refugees to register their children where they're born, and giving mothers the right to pass their nationality to their children.

"For us statelessness is not just a legal issue, it's a matter that involves the right to development," says Jesus Perez Sanchez who works for the UNHCR.

"That person that's affected by statelessness will not be able to contribute fully to that country that is hosting. So we think that it's important that as a matter of inclusion, all issues of statelessness are addressed so that all these people on the margins of society can contribute fully to society and the economy."

Back in Thembisa, Arnold is playing football with local children.

After years of struggle, he now has a lawyer helping him fight for the papers that prove he belongs here. He wants to go back to school to study computer science. He hopes having documents will lead to a brighter future.