Even Dickens did not describe harder toil. Life for the child of a tenant farmer who works the tobacco fields of Malawi is unimaginably arduous: up at 4am for several hours of labour with no breakfast, cutting into the earth with a heavy hoe.

School – if they go to school – is a brief respite. The first and often only meal of the day, maize porridge, is eaten when they get home, before more digging. Sleep is on the bare earth under a straw roof.

Now, however, British American Tobacco is facing a watershed legal action on behalf of potentially hundreds of these children and their families. Human rights lawyers Leigh Day argue that the company is getting rich while the children and their parents who do this backbreaking work are trapped in grinding poverty, which they say amounts to “unjust enrichment”.

BAT says it tells farmers not to make their children work. The lawyers say they have no choice. Most children are deprived of an education, which should be a path out, because their families struggle to afford even the small fees, exercise books and uniforms required.

“Tobacco growing, manufacture and distribution is simply incompatible with the wellbeing of a child,” said Dr Vera Luiza da Costa e Silva, head of the secretariat of the World Health Organization’s Framework Convention on Tobacco Control, a key body tackling an industry that kills more than 7 million people a year.

“Using children as workforce in any industry defeats the global fight against child labour. In the case of tobacco, children have a particularly heavy burden to carry.

“Not only do such practices hinder their ability to access education and develop in dignity, but they also expose them to what are known to be hazardous substances, such as pesticides used in tobacco crops and nicotine from tobacco leaves.”

And the gruelling life in Malawi is not what these parents hope for their children.

Junio, 36, knows that his family deserves better.

He explains he doesn’t want his children to work, but that they cannot manage without them. Three acres requires the labour of four adults. He and his wife Agnes, 30, are two. The maths isn’t hard: four of their five children are expected to help in the ways they can – Chisomo, 14, Malita, 11, Jonasi, nine, and Alinafe, seven. Only baby Gloria escapes – for now.

Many of these tenant farming families are from Phalombe, the poorest region in the south, and travel north hoping to earn a lump sum that will allow them to set themselves up back home.

A girl beside her straw home, which offers little protection during the rains

“I intend to go back if I can get enough money and build a good house and sell second-hand clothes,” says Junio. “And not suffer in life.” This is their third year. The dream is receding.

“We are failing to support our children in terms of school because of money,” he says.

Inside the hut is an empty space of bare earth, around two metres square, with a straw roof that will not even begin to keep out the torrential rains when they come. Over a piece of cord dangle a few old clothes.

Junio unfolds the monthly 50kg bag of maize (40kg after milling) that families are given.

It is more than half empty with more than two weeks to go. That is all the families have to eat – maize porridge, called nsima, cooked once in the middle of the day with leftovers for supper. They pick a green herb that grows wild to make a scant “relish” for it. In the corner of the hut is a hen. Any eggs will be sold.

The primary schools depend on what they call the development fee of 500 kwacha (50p) per term per pupil to pay for local “volunteer” teachers.

On this day, however, all the children are at home as volunteers are not working because they haven’t been paid.

Usually there are 150 pupils in a class. On the blackboard in an empty classroom are formal phrases in English. “Writing sentences in the simple future tense,” runs an instruction in chalk. None of the tenant farmer children understands a word.

Local school children

Children must supply their own exercise books and pens. Uniform is compulsory, costing 2,500 kwacha for girls (£2.50) and 1,500 kwacha for boys. At the start of the academic year, schools are being lenient. Later, children will be turned away. Then there are termly fees of 300 kwacha (30p) for exams and 200 (20p) for report cards. They sound like tiny sums but have to be borrowed, and repaid out of an income of maybe £200 for the entire year.

Junio’s eldest boy, Chisomo, was sick last Friday, he said. Illness can rack up crippling loans. But they didn’t borrow money for transport.

Instead they walked to the health centre, leaving at 4am and arriving at 6.45am. Chisomo was refusing to eat and vomiting.

When they finally saw a nurse, they were told he had a cough and given an antibiotic and a painkiller. And then they walked home again.

Malita, his 11-year-old sister, says English is her favourite subject at school, and that she would like to be a teacher.

But she doesn’t say it with conviction. Some of her friends also say they’d like to be a teacher or a nurse, adding, “But I don’t think it is possible.”

Tobacco leaves, often treated with pesticides, pose a risk to children’s health

Silika, 41, from the eastern region of Malawi, with the help of his wife and four children, made 180,000 kwacha (£180) last season. But it is not enough to go home.

After deductions, he received 140,000 (£140). At this time of year, they need to build or replace tobacco drying stands, made of large branches.

For two weeks, he and his wife walk every day to the tops of the distant hills, because the land around has been denuded of trees. They set off at 4.30am with food and water, arrive about 7.30am and get home around 4pm. If they are bringing back large branches, they can carry just one at a time.

His children go to school, but skip exam days because they can’t afford the fees. That evening the Guardian watches as Silika’s 10-year-old son takes over from his mother, Elina, 28, when she puts down her hoe to fetch his crying sister from the hut.

In sync with his father, the young boy wields the heavy hoe, made of a metal plate strapped to a wooden cudgel, raising it above his head to bring it down hard and cut into the earth. The younger ones pick up last year’s dead plants turned up by the hoes and build a pile that will be burned. They’ve done this many times before.

“At the end of the day, the amount I earn for the work I do is not fair,” says Silika. “I would like my children to attend good schools, so in the future they will be able to be self-reliant. I would like to have a business so I can support my family.”

The families are given a 50kg bag of maize to last them a month

Augustine, the deputy head of the local primary school, is unusually frank. “Most of the farmers are not sending their children to school,” he says.

The attendance register plummets in February and March during harvest time, he says. But in the mornings, after school and at weekends the children are also in the fields.

The peak age of the children working the tobacco crops is between 13 and 17.

“Most of the time they are tired and hungry,” he says. “They don’t do well. They fail exams. It is difficult for them to catch up with their fellow pupils. They repeat classes.”

In church buildings and at plastic tables placed under the shade of the trees outside, a team of lawyers and paralegals from the UK and Malawi are processing claims.

“At the minute, we have teams from Leigh Day who are in Malawi pretty much 100% of the time,” says Liberty Bridge, a 28-year-old lawyer who is part of the team.

A team of paralegals and lawyers meet farmers to process claims Photograph: David Levene/The Guardian

“We have around 20 Malawian translators and paralegals assisting with interviewing potential clients and sorting out logistics. There’s also a team of five to 10 Leigh Day staff working full-time on the case and alternating between working in the [London] office and coming out to do sign-up. We are normally in [the] country two to three weeks at a time. We are taking instructions from about 100 clients a day.”

Martyn Day, a senior partner in Leigh Day who has arrived for a visit, explains the case to a group of 25 or so farmers squatting on tree trunks.

In this remote region of Mzimba district in the north of the country, there is no internet or phone signal to speak of. Families live in isolated houses on farms. Yet somehow word gets around. There is a second talk, by Bridge, on the conditional fee agreement – the explanation that there is nothing to pay unless they win – at which point an outside funder will take 25–30%. Day gives his talk to six separate groups, all with dozens of questions.

Suzeo, a contract farmer who is a head taller than the rest, wearing a safari jacket and hat, with clean shoes, makes notes on a clipboard. He stands up to ask questions – in English. Suzeo is the local big estate owner. He is not eligible to join the case, but he supports the tenants.

Are they going to fight just for compensation or a better price for their leaf, he asks.

Both, says Day. “Unless BAT increases the price, people will have more and more claims all the time. The only way for BAT to resolve it will be to increase the price.”

What Suzeo can pay depends on the price at auction and he says it is too low.

He gestures to some ragged children nearby. “The children of the tenants. They are poor. They cannot assist their children. It is slavery,” he says.

• This article was amended on 1 November 2019 because an earlier version referred to the World Health Organization’s secretariat of the Framework Convention on Tobacco Control as the WHO. This has been corrected.