The Royal Navy is leading an international task group of warships on a security patrol through the Baltic.
Frigates HMS Lancaster and HMS Westminster, plus ships from all three Baltic states - Latvia, Lithuania and Estonia - make up the task group.
It is the first maritime patrol made up of exclusively Joint Expeditionary Force nations.
The deployment is focused on maritime security and freedom of navigation in the southern Baltic Sea and is another test of elements for the UK-led Joint Expeditionary Force, the Ministry of Defence (MOD) said.
The force is a partnership of nine northern European nations, committed to working together on a range of operations.
The MOD has described the deployment as "a concerted demonstration of Britain’s commitment to the security and stability of the region".
It comes as Foreign Secretary Dominic Raab visited Estonia for talks on regional security threats posed by Russia and climate change.
It has been billed as the biggest review of Britain's defence, foreign and security policy since the end of the Cold War.
Tanker RFA Tiderace is also deployed as part of the task group in the Baltic which is supported by aircraft from the Swedish Air Force.
Defence Secretary Ben Wallace said: "Some of the UK’s closest and most steadfast allies are found in the Baltics.
"This deployment is both the latest example of a long and proud history of defence cooperation and a clear demonstration of the capability of the UK-led Joint Expeditionary Force (JEF).
"As the first maritime patrol of made up of exclusively JEF nations, we are ensuring our ships and people are ready to operate in challenging conditions alongside our Estonian, Latvian, Lithuanian and Swedish allies."
The British ships underwent intense training in the North Sea ahead of setting sail to the Baltic.
Commander Will Blackett, Commanding Officer of HMS Lancaster, said: "It is a real privilege to command the first task group of this type and I have been impressed by the capabilities on display from our partner nations."
During the deployment, the crews will face varying weather conditions, from sunshine, to sub-zero temperatures and snowstorms.
Cover image: The task group is made up of vessels from the UK, Latvia, Lithuania and Estonia (Picture: MOD).