Three sisters are feared to have taken their nine children -aged between three and 15 - to Syria after going missing following a trip to Saudi Arabia.
Police have expressed "extreme concern" for the family's safety, while the children's fathers are said to be "distraught".
Here's what we know about the missing group:
Who has fled?
Sugra Dawood, 34, has fled with her five children.
They include (pictured from left to right below):
Junaid Ahmed Iqbal, 15
Ibrahim Iqbal, 14
Zaynab Iqbal, eight
Mariya Iqbal, five
Isameel Iqbal, three
Zohra Dawood, 33, has travelled with her two daughters:
Haafiyah Binte Zubair, eight
Nurah Binte Zubair, five
Khadija Dawood, 30, has left with her son and daughter:
Maryam Siddiqui, seven
Muhammad Haseeb, five
What are their last known movements?
The family left their homes in Bradford, West Yorkshire on May 28 to go on an Islamic pilgrimage to Saudi Arabia.
They broke off all contact with relatives on June 9.
The family's mobile phones have been turned off and Facebook and WhatsApp profiles have not been updated.
It is believed that at least 10 members of the family boarded a flight from Medina in Saudi Arabia to Istanbul, Turkey - a commonly used path into Syria - that day.
However, there is no record of two of the children - Haafiyah Zubair, five, and eight-year-old Nurah Zubair - ever boarding the flight.
They were officially reported missing when they failed to return to the UK as scheduled on June 11.
A picture of members of the family smiling at the airport before they travelled to Saudi Arabia has also emerged.
Where are they heading?
Balaal Khan, a lawyer acting for the fathers of the missing children, said there were concerns the sisters were travelling to Syria.
He said it is believed the sisters have a relative fighting for either Islamic State (IS) or another extremist group in Syria.
It is feared they have met up with him.
How have the rest of their family reacted?
The children's fathers are said to be "distraught" at their disappearance.
Naz Shah, the Labour MP for Bradford West, told the BBC that she had spoken to two of the fathers and that they were "worried out of their minds".
"The men are very, very distraught. They are confused and did not know what was happening or why it was happening."
What are police doing to find the missing family?
West Yorkshire Police has made it clear how concerned they are about the family's welfare.
The force said an investigation has been launched to establish their whereabouts and officers were "working extensively with authorities overseas to try and locate them".
What has the Government said?
In a statement, the Foreign Office said: "We are in contact with West Yorkshire Police and Turkish authorities and our ready to provide consular assistance."
Education Secretary Nicky Morgan told Good Morning Britain today that the case was "very, very concerning".
It comes after 17-year-old Talha Asmal, from Dewsbury, West Yorkshire, was reported to have become Britain's youngest suicide bomber.
And Thomas Evans, a Muslim convert from Buckinghamshire, died in Kenya fighting for extremist group al-Shabaab.