A port alert system designed to prevent people leaving the UK is "compromised", a policewoman has told a High Court judge.
Placing people's names on a "watch list" or on the police national computer did not guarantee they would be stopped, the officer told Mr Justice MacDonald.
She said not all airlines carried out checks - and that people could travel to another European country "for onward travel" without their passports being checked.
Detail of the officer's concerns has emerged in a ruling by the judge following a hearing in the Family Division of the High Court in London.
Mr Justice MacDonald did not name the officer or give her rank - but indicated she had been "in charge" of a case he was analysing.
Police and social workers had intervened after discovering that a nine-year-old girl could be in danger of being taken to Afghanistan to marry an 18-year-old man, the judge said.
The girl's mother had told social services staff that her father was planning the trip, Mr Justice MacDonald had been told.
Another judge had made both parents the subject of forced marriage protection orders.
Mr Justice MacDonald said the girl had been temporarily taken into local authority care in the wake of her mother's allegation - and he had analysed the latest stage of family court proceedings launched by social services bosses.
He said the policewoman had told him that a forced marriage protection order, which would lead to the girl's name being placed on a port alert system, was not necessarily enough to prevent her being taken out of the UK.
She had said: "In my professional experience, having subjects circulated on police national computer and/or placed on the watch list does not guarantee that they will be stopped when leaving or entering the UK."
Mr Justice MacDonald added: "The officer (had said) that the port alert system is compromised by the fact that not all airlines carry out checks, particularly if tickets are purchased close to the date of travel, and that it is possible to travel to another European country for onward travel without passport checks being undertaken.
"The officer (had said) that she has personal experience of individuals being able to leave the country notwithstanding a port alert being in place."
Mr Justice MacDonald did not identify the family involved.
But he said they were of Afghan heritage and indicated they lived in London.