Wardship proceedings in the case of the Internet twins Belinda and Kimberley have been issued in the family division of the High Court, Flintshire County Council said.
The council said the twins will remain in the care of social workers until their future could be decided at a legal hearing in Birmingham on Tuesday.
Alan and Judith Kilshaw, of Buckley, north Wales, the couple who paid to adopt the babies in America, are keeping a low profile after days of media interviews.
Keith McDonogh, Flintshire County Council's director of Education and Children's Services, said: "The twins remain safe and well in the care of Flintshire County Council.
"We have always believed that the future of these children should be determined by a considered legal process in view of all the issues involved, including complex legal, practical and international dimensions.
"This morning wardship proceedings were issued in the Birmingham District Registry of the High Court of Justice Family Division.
"A hearing will take place at the Priory Court, Birmingham, on Tuesday 23 January at 10am."
Mrs Kilshaw, 47, and her husband, a 45-year-old solicitor, have vowed to do whatever is necessary to get the twins back.
The couple, who left the hotel where they had been staying in Mold, north Wales, last night, were today preparing for the forthcoming legal battle.
Mrs Kilshaw, who has an 18-year-old daughter Cayley from a previous relationship, said: "We are fighting a very powerful strong body. It's like David and Goliath."
The couple's solicitor, Dianne Miller, had been expecting Flintshire County Council to try to make the twins wards of court.