A Royal Navy patrol ship has shadowed a Russian vessel through the English Channel.
HMS Tyne sailed on Christmas Eve to keep watch on the Smolnyy Perekov as it passed close to UK territorial waters.
The Portsmouth-based ship remained at sea on Christmas Day to monitor the Russian vessel, the Royal Navy said.
Commander Jon Browett, HMS Tyne's Commanding Officer, said: "Leaving our families at this time of year is especially difficult, but national security doesn't stop for Christmas, and as one of the duty units over the festive period we had bags packed and were ready to go."
He added: "We had members of the ship's company returning at short notice from all over the country and, hard as it is for those left at home, we are committed to keeping Britain safe all year round.
"Our families understand our role and undoubtedly make a tangible contribution to national security through their continued support.
"The UK's trade, economy and information networks depend on the sea, and this operation demonstrates the Royal Navy's commitment to protecting our home waters at all times."
HMS Tyne was due to return to Portsmouth on Boxing Day, remaining ready for tasking at short notice over the rest of the festive period.
More than 3,500 Royal Navy, Royal Fleet Auxiliary, Royal Marines and Fleet Air Arm personnel are either on duty or at short notice to respond to events at home and abroad this Christmas.
Defence Secretary Ben Wallace said: "I am grateful for the efforts of the crew of HMS Tyne, who have sailed on Christmas Eve, helping to safeguard our seas.
"This is yet another clear example of the tireless efforts of our armed forces, as they continue their tireless efforts to defend our country and our interests across the world."