High street coffee shop giant Starbucks has been caught up in a child labour row after an investigation revealed that children under 13 were working on farms in Guatemala that supply the chain with its beans.

Channel 4’s Dispatches filmed the children working 40-hour weeks in gruelling conditions, picking coffee for a daily wage little more than the price of a latte. The beans are also supplied to Nespresso, owned by Nestlé. Last week, actor George Clooney, the advertising face of Nespresso, praised the investigation and said he was saddened by its findings.

The Dispatches team said some of the children, who worked around eight hours a day, six days a week, looked as young as eight. They, were paid depending on the weight of beans they picked, with sacks weighing up to 45kg. Typically, a child would earn less than £5 a day, although sometimes it could be as low as 31p an hour.

Over the course of the investigation, Dispatches visited seven farms linked to Nespresso and five linked to Starbucks. Child labour was found on all the farms.

A human rights lawyer who viewed some of the programme’s evidence suggested both companies were in breach of international labour regulations laid down by the UN’s International Labour Organization.

“The conventions are very clear in that they don’t want children’s education to be compromised,” said Oliver Holland of solicitors Leigh Day. “If children are working 40 hours a week, there is no way they can also be having a proper education.

“These are all unsafe conditions for children essentially, and in those conditions children simply shouldn’t be working.”

Facebook Twitter Pinterest George Clooney is the marketing face of Nespresso.

Dispatches calculated that of £2.50 spent on a typical cup of coffee on the high street, the shop receives 88p. Staff receive 63p and 38p goes on tax. A profit of 25p goes to the coffee company – such as Starbucks, which has an annual global revenue of more than £20bn, and has nearly 1,000 shops in the UK alone.

After other costs are accounted for, 10p is left for the coffee suppliers, of which 1p goes to the farmer, who uses a fraction of this to pay coffee pickers.

Dispatches reporter Antony Barnett shared some of the evidence with Clooney. Last week, the Hollywood star, who sits on Nespresso’s sustainability advisory board, said: “I was surprised and saddened to see this story. Clearly this board and this company still have work to do. And that work will be done. I would hope that this reporter will continue to investigate these conditions and report accurately if they do not improve.

“The check and balance of good corporate responsibility lies not just with the company itself but also independent journalists like Mr Barnett to hold everyone’s promise to account.”

Nespresso’s chief executive, Guillaume Le Cunff, said: “Nespresso has zero tolerance of child labour. It is unacceptable. Where there are claims that our high standards are not met, we act immediately. In this case, we’ve launched a thorough investigation to find out which farms were filmed and whether they supply Nespresso. We will not resume purchases of coffee from farms in this area until the investigation is closed. Any issues we uncover will be dealt with diligently and firm action will be taken.”

Starbucks also said it had a “zero tolerance for child labour anywhere in our supply chain”. It told Dispatches: “We’ve launched a full investigation into the claims brought by Channel 4, carried out in partnership with a leading third-party auditor.”

Starbucks has since said that its investigation confirmed “we have not purchased coffee from the farms in question during the most recent harvest season”.

“We remain concerned and are taking action due to the fact that these farms were verified in 2019 against our ethical sourcing standards, which are the most comprehensive in the coffee industry,” said Michelle Burns, Starbucks global head of coffee.

Barnett said: “It’s great that George Clooney supports our investigation but if he is serious about sorting out this issue, he needs to make sure Nespresso puts its money where its mouth is. It’s far too easy to announce an investigation and halt supplies from these regions but this will further punish the farmers and desperately poor families who rely on them. The reason these kids are working is that their parents – and the farms they work on – are not paid enough.“

Dispatches will air on Monday at 8pm.

• This article was updated on 1 March 2020 to include a statement from Starbucks that was received by the Guardian after publication. The headline was also amended in light of the statement.