Jakarta: The four richest Indonesians are wealthier than the poorest 100 million people in the country, a study has found, highlighting how huge numbers have been left behind as the economy booms.

Oxfam said that President Joko Widodo had failed to fulfil pledges to fight inequality and called on the government to urgently increase spending on public services and make corporations and the wealthy pay more tax.

Indonesia has enjoyed an economic boom that has reduced the number of people living in extreme poverty but the gap between rich and poor has risen faster than in any other Southeast Asian country in the past 20 years, the Oxfam study said.

‘The benefits of growth have not been shared equally, and millions have been left behind,’ said the research, which was released earlier this week.

‘The widening of the gap between the rich and the rest is a serious threat to Indonesia’s future prosperity. If inequality is not tackled, then reducing poverty will be much more difficult, and social instability could increase.’

In 2016, the collective wealth of the four richest Indonesians, all men, was $25 billion, Oxfam said. This was more than the total wealth of the 100 million poorest, out of a population of 255 million, it said.

According to Forbes rich list, the wealthiest included three tobacco tycoons — brothers Michael and Budi Hartono, and Susilo Wonowidjojo.

The study said inequality has been rising since the 1997 Asian financial crisis, which led to the introduction of more free market policies.

This shift allowed those at the top to capture the greatest share of the benefits of years of strong growth, it said. The study pointed to rising inequality within cities, and between urban and rural areas.

Oxfam said Indonesia’s taxation system had failed to play a role in redistributing wealth as it was not collecting nearly as much revenue as it should, while an underfunded education system was stopping many from getting better jobs.

In January the government launched a major programme to tackle inequality that included measures aimed at helping poor farmers and fishermen, improving the tax system and providing more opportunities for vocational training.