India has the most slaves in the world, with over 18 million people trapped in debt bondage, forced into marriage, sold to brothels or born into servitude, according to a global slavery index.

40 per cent of the world's estimated 45.8 million slaves are in India, although the scourge exists in all 167 nations surveyed by the Australian-based group Walk Free Foundation.

Fiona David, head of global research at Walk Free, said while estimates of slavery had risen by 15 per cent in India from the previous figure due to better data collection, government efforts to curb such exploitation had also improved.

"The incidences of slavery in 2016 is larger than we thought. This doesn't necessarily mean the numbers have gone up since the last Global Slavery Index, but more because of a greater improvement in our measurements," David said.

"But what is new is that the Indian government is taking really exciting steps to bring all the different pieces of legislation together into one anti-trafficking act. It's a huge step forward."

The index found India with a population of 1.3 billion had the largest number of slaves in absolute terms at 18.35 million, followed by China, Pakistan, Bangladesh and Uzbekistan.

India, however, ranked fourth in the index in terms of prevalence of slavery as a percentage of the population - at 1.4 per cent - after North Korea, Uzbekistan, Cambodia.

The third edition of the index was based on interviews US pollster Gallup conducted with about 42,000 people globally - 14,000 of them in India.
All forms of slavery were prevalent in India, it said, including inter-generational bonded labour, forced child labour, commercial sexual exploitation, forced begging, forced recruitment into non-state armed groups and forced marriage.

Respondents admitted to being made to work to repay a loan taken by themselves or another family member.

Girls and women described being forced or duped into prostitution by traffickers and brothel owners, many of them locked in a room and repeatedly raped.

Domestic workers spoke of a lack of freedom, long working hours, little or no wages and physical and sexual abuse. Street beggars said they were made to beg by organised criminal gangs.

The report said a preference for sons in India had led to the widespread, yet illegal practice of aborting female foetuses - resulting in fewer females in some parts of the country and fuelling the trafficking of girls and women for brides.

"It is reported that in some instances, girls are forced into marriage and then used as unpaid labourers - local day labourers cost $140 for a season but a bride can cost only $US100 as a once off payment," the Global Slavery Index report said.

In Indian states such as Kashmir, Jharkhand, Assam, Odisha and Chhattisgarh, children were being forcibly recruited by opposition groups as informers or trained to fight, it noted.