Sex offenders are entering the UK after borders to migrants from eastern Europe were opened, a police chief has warned.
Terry Grange has been supported by Polish community members
Terry Grange, the Dyfed-Powys chief constable and Association of Chief Police Officers' spokesman on child protection, said people may be at risk.
He told BBC Wales' Dragon's Eye there was not a level playing field with the open borders policy.
He said other countries did not have Britain's sophisticated policy of monitoring sex offenders.
In comments supported by representatives of the Polish community in Wales, Mr Grange said some offenders would be leaving those countries bound for the UK, without appearing on any register.
"There will be people coming into this country with criminal records and sex offenders too - guaranteed," he said.
"The reality is we don't know who they are and we should - no matter where they come from."
Mr Grange said other countries should set up a sex offenders register and then share that information if suspects planned to leave.
"It will take five years, if they choose to start now about sex offenders registers," he added.
"Our partners in Europe should be looking at how they keep information and should be passing it around. We do. We can tell everyone else in the world - and they should be able to tell us."
Asked whether the risks were too great, he replied: "In terms of sex offenders, yes. They hit to the heart of things... our children our elderly and female population being put at risk when we could do something about it.
"What we can do is seriously address the issue of criminal records across Europe, harmonising the way they are recorded. And be absolutely serious about controlling the flow of information when people leave."
"Given the choice I wouldn't let anyone into this country who has a particular form of criminal record and that is sex crimes. But that is only a personal opinion"
Terry Grange praised the UK government for trying to take up the issue 18 months to two years ago. Even so, he said, nothing had happened at a European level.
He said while the country had benefited from migration, authorities should know the background of people who moved to the UK.
One Polish national was jailed for life two years ago for raping a woman near Llanelli. Only afterwards did it emerge that the man, Josef Kurek, had served 13 years in jail in Poland for rape and other sex offences.
Mr Grange said it was likely another rapist of this kind was in the UK.
"There is bound to be one - and more. That is the nature of the beast - a very small portion are dangerous and I'd like to know who they are so that we could try and do something about it."
Reacting to this comments, Halina Ashley of the Polish-Welsh Mutual Association, said it was of concern to them that "there are elements coming in that are not desirable".
"There are people also who have criminal background and postponed sentences in Poland and are here biding their time. They are here, we know that," she said.
"This is why we approached the police, unfortunately the answer was that nothing could be done."
Dragon's Eye is on BBC 2W at 2100 BST on Thursday, and at 2235 BST on BBC 1 Wales.