Thousands of people have been trafficked to the UK and kept in conditions of modern slavery in the past year, according to the latest police figures.

Statistics released by the National Crime Agency (NCA) show the number of potential victims of trafficking last year increased by 22% on 2012, rising to 2,744 people from more than 86 countries, of whom 602 were children.

The report drew a critical response from Andrew Wallis, chief executive of anti-trafficking NGO Unseen, who said it demonstrated that the UK still did not understand of the scale of the crime.

But Aidan McQuade, director of Anti-Slavery International, said the increase in the number of reported victims could partly be attributed to a better understanding of the issue by the government and authorities.

Eastern Europe remains a primary source of victims. The highest number of people trafficked into the UK came from Romania and most of them were sexually exploited. Poland was the most likely country of origin for people facing labour exploitation.

Of all known victims of labour trafficking, 78% were European Economic Area nationals legally working in the UK.

There has been a significant rise in the number of UK victims. British people made up 7% of the overall trafficking figures last year, up from 4% in 2012. Nearly 90% of UK children identified as potential victims of trafficking by the NCA had been sexually exploited, an increase of 250% on 2012.

Although the figures were appalling, McQuade said, “paradoxically this is probably good news because it indicates a more effective scrutiny of the problem, but saying that it is still a dreadfully high number, which indicates that more attention needs to be paid to getting to grips with the push factors that are leading to people being identified as trafficking victims in the UK.”

“We should also bear in mind that these figures almost certainly underestimate the actual numbers of those affected, for instance in the sphere of forced domestic work, which the government seems to be addressing by simply ignoring its existence,” he added.

The NCA admitted that, despite improvements in identifying victims, the actual number of people trafficked in the UK is likely to be higher than those recorded. It acknowledged that some victims may not be identified by the authorities that encounter them, such as women who have been sex trafficked into prostitution.

Wallis was concerned about the gap between the reported figures and the likely real ones. “These figures and data are already nine months out of date and the report is littered with phrases like, ‘it is not clear why’ and ‘lack of information regarding’. In 30% of all cases the location of exploitation was recorded as unknown – which goes to highlight that the UK still remains ‘data blind’ and lacks the necessary understanding of the true scale and nature of this crime.

“Of particular concern is the explosion in the number of children identified as trafficked, especially UK nationals, and the acceptance that the true figure is much higher as we are beginning to understand the true scale of situations like Rotherham, Rochdale, Telford and Oxford, to name but four.”

The NCA admitted that people who were trafficked into the UK for criminal exploitation, such as being forced to work in cannabis factories or begging gangs, were still seen as criminals rather than victims in some cases.

The report indicated that some victims had been sold on several times after entering the UK, for amounts ranging from £200-6,000. Nigerian traffickers, who arrange for documentation and travel for women who believe they are coming to the UK for legitimate work, are demanding up to £50,000 from their victims, who are forced into prostitution to pay off their debts.

The NCA figures were published as the Home Office prepares to put its modern slavery bill to the Lords this year.

“Modern slavery is an appalling crime that has no place in today’s society,” Karen Bradley, the modern slavery and organised crime minister, said. “Yet these figures show that it is taking place here – often out of sight – in shops, fields, building sites and behind the curtains of houses on ordinary streets.”