The Finnish parliament has approved new legislation aimed at curbing human trafficking in the sex trade.
Under the new law, a client paying for sex with a prostitute could face six months in prison if the prostitute is a victim of human trafficking.
It is estimated that more than a third of all prostitutes in Finland come from abroad, mainly Russia and the Baltic states, and are controlled by pimps.
Police say the law will discourage prostitution by frightening clients.
But police and magistrates will have to prove that clients knew the prostitute was forced to sell her services, in order to secure a guilty verdict.
"You will have to show that there was no direct contact between the prostitute and the customer but that there was something or somebody between these two persons," said an adviser to Justice Minister Leena Luhtanen, Janne Kanerva.
He added that culprits were more likely to be fined than jailed.
The parliament rejected an alternative law, modelled on Swedish legislation, which would have made buying any sexual service a criminal offence.