Officials have no idea what has happened to 2.3 million people who have registered as temporary residents since 2014, Trouw reported on Thursday.

People planning to do seasonal work in the Netherlands for less than four months are required to register as ‘non-residents’ but do not have to give an address. In return they are given a bsn number, which allows them to work.

However, no-one knows if they are still here, are working or have actually left the country, Trouw said. And this, experts say, makes it impossible to find out if they are living in decent conditions or are being exploited at work.

‘We have created an invisible group,’ Tilburg University professor Conny Rijken told the paper. ‘Given that we know labour migrants… are vulnerable to different forms of fraud, we should at the very least be alert to this.’

People planning to live in the Netherlands for more than four months are required to officially register with their local authority using a formal address.

The RNI register also includes some two million Dutch nationals who live abroad but who have economic ties with the Netherlands.