A High Court judge has raised concerns after two English social workers made a five-day trip to Hungary to assess the Hungarian family of a baby born in England.
Mr Justice Moylan said no inquiries had been made as to whether it was permissible under Hungarian law for foreign social workers to make such assessments.
And he said the legality of such assessments was an important issue which had to be addressed before social workers made trips abroad.
Detail has emerged in a written ruling after the judge analysed the boy's future at a hearing in the Family Division of the High Court in London.
He said the local authority involved was Leicester City Council - but he did not identify the little boy or the social workers.
Mr Justice Moylan said the baby had been born in England in March 2013. His mother, who had arrived in England in 2012, was Hungarian. His father was also thought to be a Hungarian living in Hungary.
Mr Justice Moylan said the social workers had travelled to Hungary in late June 2013.
"The local authority sent a request to the Hungarian Central Authority for a considerable volume of information and documents," he said.
"The local authority also sought the assistance of the Hungarian Central Authority for the purposes of a proposed assessment of the mother and other family members to be undertaken by two social workers from England travelling to Hungary.
"No response was received to this email but, nevertheless, two English social workers went to Hungary from 30 June to 4 July 2013 for the purposes of carrying out assessments of the mother, the father and extended family members.
"Before this took place, no other inquiries were made and no investigation undertaken to ascertain whether this was permissible or lawful under Hungarian law.
"I acknowledge that, when in Hungary, the social workers spoke to a number of Hungarian professionals, none of whom appear to have raised any concerns."
He added: "Nevertheless ... This is an important issue which must be addressed before a social worker from this jurisdiction undertakes any professional work in another country."