David Cameron has been urged to sign up to a directive tackling human trafficking during the weekly session of Prime Minister's Questions.
Harriet Harman - in her last session as acting Labour leader on 15 September 2010 - said he should not let Tories who were "irrationally hostile" to Europe "stand in the way of stopping human trafficking".
Mr Cameron said human trafficking was an "evil trade" but that the UK was already taking all the steps outlined in the directive.
He added that the previous government signed too many directives without thinking through the consequences.
He was also questioned by Conservative backbencher Julian Brazier on the issue of child poverty, who said a "hand out society" needed to be replaced by a "hand up" one.
The Prime Minister told the house that the key cause of child poverty was children growing up in households where nobody works.
He opened the session by paying tribute to Ms Harman saying she was "the third Labour leader I've faced...but probably the most popular".
Labour will elect a new party leader during the party conference season, before Parliament returns in October.