Pakistan accounts for 612 of over 1,400 cases of forced marriage reported in Britain last year, according to data released on Thursday, a number which campaigners say is the "tip of the iceberg" as the practice remains largely hidden in the United Kingdom.

Britain's Forced Marriage Unit (FMU) recorded 1,428 cases in 2016 ─ a small increase on 2015's figure of 1,220 ─ through its public helpline and email service. The unit also said it had received the highest number of calls to its helpline since 2012.

Many cases involve girls from South Asian backgrounds, but the FMU said it had handled cases involving more than 90 countries since 2005.

Bangladesh accounts for 121 of the cases handled, India (79), Somalia (47), Afghanistan (39), and Saudi Arabia (16).

Only 157 of cases involved potential or actual forced marriage happening in Britain.

Around a quarter of cases in 2016 involved children under 18, but most victims were between 18 and 24 years old.

Activists said they were encouraged by the rise in recorded cases, saying it was likely due to more people reporting the crime because of greater awareness.

"When you get figures like this, the first reaction is, 'this is absolutely horrendous'. But the positive thing is people feel that they're able to report it," said Aneeta Prem, founder of Freedom Charity, which campaigns on forced marriage.

Forced marriage is illegal under British law, even if carried out abroad.

Campaigner Jasvinder Sanghera, chief executive of Karma Nirvana, a charity which supports survivors of forced marriage, said despite the increase in recorded cases, the practice remains a hugely under-reported issue.

"The victims are extremely isolated and the people who are abusing them are their nearest and dearest ─ family members. So you can imagine how difficult it is to report it in that environment," said Sanghera, who escaped from a forced marriage as a teenager.

"We're all dealing with the tip of the iceberg because (forced marriage) is completely under-reported and we have got a lot to do," she said.