More than 7,100 suspected victims of modern slavery were identified across the UK in 2018, with Romanian nationals comprising the largest victim group, according to a national helpline.

Labour exploitation – the majority of which was identified in car washes, beauty parlours, construction sites, hotels and on farms – accounted for the largest number of suspected modern slavery cases, with London the location for the highest number of suspected victims (1,477), found the helpline’s second annual assessment, published on Thursday.

Anti-slavery charity Unseen UK, which set up the helpline in 2016 and its sister app last year, said calls increased from approximately 40 a week in October 2016 to more than 150 a week in December 2018. Calls increased by 62% from 2017 levels and webforms by 86%, the group said.

Contact came from members of the public, police, activists, healthcare and legal professionals, as well as from slavery victims themselves.

Unseen’s CEO, Andrew Wallis, described the rise in calls to the hotline as “a testament to the scale of the problem of modern slavery in the UK today”.

“The numbers of potential victims indicated are just the tip of the iceberg. But growing public awareness of the signs of slavery, and of the helpline as a resource, mean that we are able to help more people to freedom than ever before, working in collaboration with the police, businesses and other partners, while the data coming out of the helpline is being used to further understand this crime and how to combat it,” said Wallis.

Although Romanians made up the highest number of suspected slavery victims, the number of Vietnamese nationals jumped 249% between 2017 and 2018, according to the report. Pakistani nationals were the next highest group, with a 105% increase in reported cases.

The global slavery index estimates that as many as 136,000 people may be living in modern-day slavery in the UK, a figure 10 times higher than the government’s estimate.

Despite the rise in calls to the hotline – which Unseen stresses is probably due to growing awareness of slavery rather than an increase in actual incidences – public reports of suspected slavery cases do not often translate into arrests or prosecutions.

Although one quarter of all suspected labour abuse cases related directly to car washes in 2018, according to the report, investigating authorities have previously warned that much of the information they receive is extremely vague, such as “the workers didn’t look happy”.

The government has also been criticised for failing to provide adequately for people who have been formally identified as slavery and trafficking victims. A high court is expected to hand down a judgment in June on the unlawfulness of a Home Office policy that cuts all financial, psychological and housing support to survivors after 45 days.

Jasmine O’Connor, CEO of Anti-Slavery International, said the UK government’s “hostile environment” policy towards migrants had helped create space for slavery to flourish.

“Slavery thrives when we allow discrimination and a hostile environment for migrants. If someone is treated as a criminal when they don’t have the right papers, they are easy prey for traffickers, who can act with impunity,” she said.

“We also need to consider how our demand for cheap goods and services contributes to employers taking advantage of workers and can become a slippery slope to slavery. We need to tackle the bigger picture and think about how we all play a role.”

Unseen warned of an increasing number of potential slavery cases related to churches and other places of worship as areas for recruitment, or where exploitation may have taken place. The report found that labour exploitation in recycling and waste facilities, and in the transport sector, was also on the increase.

For the first time, the charity also identified 2,171 suspected exploiters in 2018. While most were male, women comprised a significant proportion (41%) in domestic servitude cases. More than half of the exploiters (54%) were employers, but recruiters, family members, intimate partners, or foster parents to the potential victim were also among those identified by the report’s authors. The largest group of potential exploiters were Romanian (77), with English (64) and Vietnamese (54) exploiters not far behind.

“No other organisation is putting this information out there: exploiters’ nationality, their gender, how they recruit and their methods of control,” said Unseen’s executive director, Justine Currell. “All that information helps [identify exploiters and] prevent modern-slavery from occurring in the first place.”

• This article was amended on 30 April 2019 to make clear that Unseen UK reported a 62% increase in calls between 2017 and 2018, not 2016-18.