Italian humanitarian organisations have accused French police of falsifying the ages of teenage migrants so they can send them back across the border between the two countries.

Seven charities, including Oxfam Italy and the Catholic organisation Caritas, claim that in recent weeks French officers deliberately altered birth dates on documents to make it appear that the young migrants were older than 18.

The alleged tactic circumvents international rules that say that under-18 year olds must be given protection and allowed to cross borders to reunite with family members.

The rule for adults, by contrast, is that they must apply for asylum and remain in the first EU country they reach – which in the vast majority of cases means Italy, Spain or Greece.

The falsification of documents allegedly took place near the Italian town of Ventimiglia, on the border between the French and Italian Rivieras, where tens of thousands of migrants and refugees, many of them unaccompanied minors, have tried to cross in recent years.

“The French police falsify the documents of minors who try to cross from Italy into France,” said Daniela Zitarosa, from the charity Intersos.

“We have the proof – many dates of birth were modified in official documents. Unfortunately this has become routine.