Theresa May has paid tribute to the "valiant" servicemen and women who help keep Britain safe and said she is "immensely grateful" for the sacrifice of those who cannot be home for Christmas.
In her seasonal message to the troops, the Prime Minister also recognised the sacrifice of parents and children who often have to change their school or job and cope with long periods apart from their loved ones.
She said: "I want to send a message of thanks to every member of our armed forces and your families, wherever you are in the world this Christmas.
"Earlier this year I joined the centenary commemorations for the Battle of Passchendaele.
"Gathered in Flanders, we remembered the hundreds of thousands of young men who gave their lives on that battlefield in the cause of freedom.
"Through a century of great change since, the high standards and devotion to duty of our armed forces have remained constant.
"Today, as we face new threats, your work continues to make the world a safer place."
Mrs May recognised the work done by Britain's armed forces training Iraqi forces as they fight Islamic State (IS), the RAF's bombing campaign which has helped shrink the militants' influence in the region, and the Navy's work bringing disaster relief to those hit by Hurricane Irma in the Caribbean.
And she highlighted the work of troops who were twice deployed on British streets following terror attacks at home.
"Whenever you are called upon - regulars or reserves - you always give of your best and inspire us all with your service," she said.
The PM drew attention to the military covenant, which ensures servicemen, women and their families are treated fairly.
"Partners and children are often called on to make huge sacrifices of their own - from a change of school or job, to coping with extended periods of separation," she said.
"That separation is especially difficult at Christmas time, and we should all be immensely grateful for that sacrifice."
Mrs May added: "At the Passchendaele memorial service at the Menin Gate this July, those gathered sang the Ypres hymn O Valiant Hearts.
"This Christmas, as people across the United Kingdom celebrate this special time of year with their families and friends we will do so secure in the knowledge that the valiant hearts of our servicemen and women, many far away from their own loved ones at this special time of year, are working to keep us safe.
"Thank you all for your service and I wish you a very happy Christmas."