Even if your ex has Parental Responsibility, under the law, they cannot just take your child(ren) without your agreement.
Under the Children Act 1989 (Section 8) you can apply for a Prohibited Steps Order (PSO) to prevent specific actions, such as removing your child(ren) from your care or school without permission. If the situation is urgent, call the police on 999.
Taking a child out of the country without the necessary permission is a serious criminal offence under the Child Abduction Act 1984 , if your ex has taken your child(ren) to another country please follow these steps:
1. Call the police.
2.Call the Foreign, Commonwealth & Development Office to report the incident.
3. Call Reunite, a charity specialising in child abduction cases.