The numbers of migrant workers in parts of north Wales is unknown officials have admitted.
Members of the Blaszskiewicz family from Poland have settled in Wales
North Wales Race Equality Network said the number of migrant workers in Wales could be five to 10 times higher than the official figure of 12,670.
Half of the local education authorities in Wales also say Welsh Assembly Government grants to teach non-English speaking pupils do not cover costs.
The assembly government said it was examining funding for language needs.
In the latest Accession Monitoring Report for May 2004 - September 2006, 12,670 migrant workers were registered in Wales.
Manu Patiar, chair of North Wales Race Equality Network, told BBC Wales' Week In Week Out programme these official figures were unreliable because the information they were compiled from was not detailed enough.
"All we have to go by is the National Insurance Number, or the Worker Registration Scheme," she said.
"They do provide some statistics, but what they don't provide is where these migrant ethnic communities come from... Which nationality, which local area do they work in?
"So, I mean, even the sort of figure you are quoting, the numbers could be five or 10 times higher, it's hard to say."
Wrexham local authority told the programme the figure of Polish migrant workers in their area was anything between 4,000 and 15,000, but the lack of clarity on figures was a problem for the whole of the UK.
Aled Roberts, leader of Wrexham Council called for a more strategic approach at national level to the issue.
"It isn't just a problem for Wrexham, I think it's a problem for the whole UK because it's quite clear there are no means of recording migrant workers," he said.
"We're coping well, but there's pressure on certain services - we had in excess of 80 children register from outside the UK in our schools in September, 50 were from Poland."
In a statement, the assembly said it was in the process of commissioning research to provide more accurate estimates of migrant workers in Wales.
It admitted the figure of 12,670 registered workers may not include self-employed workers, who are not required to register with the Workers Registration Scheme in Wales.
Wojciech Blaszskiewicz is one of thousands of Polish migrants
One Polish couple who first arrived in Wales in 2004, have decided to stay on.
Wojciech and Karolina Blaszskiewicz, who now have a baby, Julia, said they will stay in Wrexham.
"I don't need to really go back to Poland because I've got my mum here and sister, and family now, so my family is here, my home is here," said Mrs Blaszskiewicz.
Week In, Week Out also reports that Welsh Assembly Government education grants to teach newly-arrived non-English speaking pupils, do not cover the actual cost of staff needed in half of the local education authorities in Wales.
In a statement, the assembly government said funding for staff in this area had increased by over £1m since 2004, to a total of £5m.
But they recognised the number of migrant workers from the EU coming to Wales in the near future was set to increase, and were looking at finding additional funding to support their language needs.
Week In Week Out is broadcast at 2235 GMT on BBC One Wales on Tuesday, 6 March.