The number of modern slavery offences recorded by police has increased by more than 50% in a year, figures reveal, as the UK government published a plan to tackle the crime in its own supply chains.

Police recorded 5,144 modern slavery offences in the year ending March 2019, data published by the Office for National Statistics (ONS) shows, a 51% rise over the previous 12 months, when 3,412 were recorded.

There were 6,985 potential victims of modern slavery referred through the UK’s national referral mechanism (NRM) in the year ending December 2018, a rise of 36% in a year, the figures show.

This coincided with a 68% increase in calls and submissions to the modern slavery helpline over the same time period.

Modern slavery can take many forms including people trafficking, forced labour, servitude and sexual exploitation.

The ONS said the number of victims and offences was likely to be higher. Many victims … do not self-identify, which means that quantifying the number of victims is challenging,” it said. There may be as many as 136,000 modern slavery victims in the UK, according to earlier estimates.

The ONS analysed figures from the police, the Crown Prosecution Service, the modern slavery helpline, the NRM, the Salvation Army and others.

Helen Ross, of the ONS’s crime and justice centre, said: “This is the ONS’s first attempt to bring data sources together on modern slavery to help us better understand both the nature of this terrible crime and the potential demand on support services.

“While there is no one source or method available which accurately quantifies the number of victims in the UK, evidence suggests there have been improvements in identification since the introduction of the Modern Slavery Act in 2015. Yet this is only part of the story, as many cases remain hidden and unreported.”

Almost a quarter – 23% – of the potential victims referred through the NRM in the year ending December 2018 were UK nationals.

The figures came as the government claimed to have become the first in the world to publish a modern slavery statement, which sets out how it is tackling modern slavery in its supply chains.

The statement outlines actions taken to ensure taxpayers’ money does not fund criminal exploitation. Ministerial departments will publish individual statements from 2020-21.

Measures taken include engagement with around 400 suppliers to implement effective due diligence practices and training more than 250 government commercial staff.

Thousands of businesses are required to publish annual modern slavery statements under the Modern Slavery Act 2015.

Theodore Agnew, a Cabinet Office minister, said: “As a government, we are committed to driving out modern slavery in all forms, so it is crucial that we make sure it’s nowhere to be seen in the supply chains which help provide our vital public services.”