HMS Albion has left Plymouth for five months in the Mediterranean to lead a NATO task group.
Speculation is continuing about the possibility of cuts to the Royal Marines in the defence review which is now underway.
The amphibious assault ship, HMS Albion is deploying to become the NATO flagship, heading the Standing NATO Maritime Group 2 (SNMG2).
Speaking before the deployment, Captain Tim Neild, HMS Albion commanding officer, said: "It is a huge privilege for us to fly the flag of the NATO task group commander in such a high profile deployment.
"This symbolises the importance that the UK and the Royal Navy places on our role in NATO and in assuring the collective defence and security of NATO allies and partners."
This new tasking fulfils a vital contingency role for NATO in the Mediterranean.
HMS Albion will undertake presence and security operations as well as conducting multi-national exercises aimed at increasing the way the navies of the task group work together.
The commander of SNMG2, Commodore Mike Utley, said before Albion set sail:
"Our role in NATO sits right at the heart of British government policy to reinforce our commitment to international partners and their broader security.
"What better way for the UK to contribute than to lead this task group from HMS Albion - I am immensely proud to command such a highly professional and internationally diverse force."
HMS Albion is returning to operations after a two-year period of extensive modernisation and a programme of training and sea trials.