Across the world, an estimated 5.1 billion people – two-thirds of the global population – are being failed by the justice system, a study has found.

But providing universal access to basic justice could save the global economy billions of dollars every year, as lost income and stress-related illness due to seeking legal redress can cost countries up to 3% of their annual GDP, according to a report published today by the Task Force on Justice.

The report said that of the 5.1 billion people with no meaningful access to justice, an estimated 1.5 billion – one in five people worldwide – have been left with justice problems they cannot solve, whether that involves a land dispute, consumer debt or being the vicim of crime.

An estimated 253 million people live in extreme conditions of injustice and are deprived of any meaningful legal protections: 40 million people are modern-day slaves, 12 million people are stateless, and 200 million live in countries or communities where levels of insecurity are so high that they are unable to seek justice, the team found.

Lacking legal identity or work documentation could lead to being deprived of justice, or it could result from living in a society where right to justice is simply denied, said David Steven of the Pathfinders for Peaceful, Just and Inclusive Societies, which serves as the Task Force secretariat.

“Justice is a public service that isn’t treated as a public service, and that’s true in the UK and more true in other countries,” said Steven.

“The critical thing here is that we just assume the world’s justice systems will only meet the needs of a very few people, just like we assumed with education in the 19th century. As we made the case for investing in education for girls as something important and cost-effective, we now have to demonstrate the benefits of doing the same to justice systems and maximise that momentum for change.”

The study, which is being launched on Monday at the World Justice Forum in the Hague, found that injustice affects all countries, but women, children, people with disabilities, minority ethnic communities and the poor are worst affected. Across the world, half of all women questioned said they believed it was pointless to report a case of sexual harassment to the police, while two in five of all modern-day slaves are children, the report claims. Task Force on Justice is a team of justice leaders and experts from civil society, governments and the private sector.

The study underscores the massive task ahead for countries to meet sustainable development goal 16 by ensuring universal access to justice by 2030, said Pakistani human rights campaigner and Task Force co-chair Hina Jilani.

“The justice gap is both a reflection of structural inequalities and disparities in power, and a contributor to these inequalities. The burden of injustice falls all too heavily on women, children and other vulnerable groups who face the hardest struggle to access the justice system and exercise their rights,” said Jilani.

Until now, there was insufficient data on global justice systems and how poorly they were performing, said Steven. The Task Force compiled people-centred figures – including specialist surveys on legal needs for women, children and vulnerable groups – with national statistics and independent surveys to come up with its findings.

When governments invest in better justice, conflict and instability go down, according to the report. It said that after the Guatemalan government rebuilt the justice system to better combat impunity and tackle corruption, homicide rates declined by 5%. In the UK, the Citizens Advice Service generates more than £10 in wider social and economic benefits for every £1 invested.

“Lack of justice is stopping countries from reaching their economic and social potential. Injustice feeds further injustice. It creates conditions for populist and extremist movements to prosper,” said Sierra Leone’s first female attorney general and justice minister, Priscilla Schwartz, who also serves as Task Force co-chair.

The cost of providing universal access to basic justice services is cheaper than healthcare or education, the report found. In low-income countries, basic justice would cost $20 (£15) per person, while universal primary and secondary education costs $41, and healthcare costs at least $76, the report found.

Justice reforms in the past have too often focused on benefiting institutions rather than the people using them, said Steven. Changes today should focus on data and evidence to steer reform, working with the education, employment, healthcare and housing sectors – despite the upset that might cause to elites and others who benefit from the status quo.

In Sierra Leone, reform of a colonial-era justice system will require extensive input from a population that regularly sees five- to 10-year delays for simple cases to be heard in formal courts, said Schwartz.

“We are still in two worlds from a legal perspective, and that is what we are trying to change: 60% of the population live in the provinces outside of Freetown, where we have local courts that depend on a tribal head applying customary law. In the capital, we have lawyers, judges, wigs, gowns and English law courts,” said Schwartz.

“We have commissioned researchers to go out in the next two months to the provinces and ask people, NGOs, communities, what they need: do they prefer local courts as they are faster, or do they prefer going to judges? These changes will affect their lives, so we need to include their views.”

In Uganda, where 85% of all lawyers are based in and around Kampala, the rest of the country must make do with very few legal services, said lawyer Gerald Abila, whose non-profit organisation Barefoot Law makes use of specialist legal technologies – some of which are powered by artificial intelligence – to bridge the gap.

“Most people [in Uganda] do not even know that they have justice needs or issues or how they can resolve them,” said Abila.

“Our goal is to make access to legal information and support available to 2 million people across Uganda by 2021, and to scale that to 50 million people across Africa by 2030,” he said.