A High Court judge today launched a nationwide hunt for four missing children.
Mr Justice Roderic Wood said Jessica, Tomas, Eva and David Palacin Jones were thought to be with their mother, Jennifer Jones.
He said the children - aged between 14 and eight - had gone missing from Ms Jones' house in Llanelli, Carmarthenshire, and "must be found".
Mr Justice Roderic Wood said the children were at the centre of a legal dispute between Ms Jones and her estranged Spanish husband Tomas Palacin Cambra.
He said they had been with Ms Jones but should have been returned to their father's home in Spain.
"I am very concerned about the children," said the judge, at a hearing in the Family Division of the High Court in London today.
"They must be found. I ask anyone who knows where they are or might have seen them to tell this court or the local police."
The judge said the children could be identified and images of them published.
Lawyers representing Mr Palacin Cambra, 52, said Ms Jones, 46, taught English as a second language and was thought to be with her partner, John Williams.
They said Ms Jones speaks with a strong Welsh accent and is slim, about 5ft 7in tall and has strawberry blonde wavy hair and brown eyes. Mr Williams is in his late 40s or 50s and is said to be taller than Ms Jones and well-built.
Lawyers said Mr Jones has family in the Swansea area and the group could be at a port or airport.
They said Mr Palacin Cambra lives in Majorca.
Lawyers said Jessica 14, Tomas 12, Eva, nine and David, eight, lived with their father in Majorca.
Mr Justice Roderic Wood said the youngsters were "vulnerable".
He said last week another High Court judge, Mr Justice Hedley, had decided that Ms Jones had "abducted" the children from Spain, where they were in the care of their father.
Ms Jones had been ordered to return the children to Spain by midnight on Friday October 12 but had not done so.
Late on Friday she had been ordered to attend a High Court hearing in London on Monday.
"The mother came to my court yesterday and argued through her barrister that the order of Mr Justice Hedley ... should be set aside," said Mr Justice Roderic Wood.
"I declined to set that order aside."
He added: "I was also concerned that the mother might abscond overnight and so I, with a heavy heart, ordered that the children were to be immediately removed from the mother's care and placed in the care of the local authority for a short period of time so that they could be returned to Spain without delay.
"When the police came to remove the children very early this morning from the mother's house in Llanelli, Wales they found the mother and the children had gone."
He added: "All these children are now missing. The mother removed them from her home in the middle of the night. Her and her children's whereabouts are unknown. I do not know the mother's intentions."
The judge said Ms Jones had "abducted" the children before.
"This is the second time this mother has abducted the children from Spain," he said. "The last time she did so was in 2009."
Lawyers for Mr Cambra released photographs of the children - and their 16-year-old sister Sara. They said Sara looked like her mother.