British fighter jets have flown their first mission over Poland as part of operations to protect NATO's eastern flank.
It followed repeated violations of NATO airspace by Russian drones and planes in recent weeks.
UK Defence Minister John Healey said the Eastern Sentry mission had sent "a clear signal" that NATO airspace would be defended.
"NATO airspace will be defended," Mr Healey said in a statement.
Two Royal Air Force Typhoons took off from a British military base in England on Friday night to patrol Polish skies and defend against aerial threats from Russia, including drones, the statement said, adding that they returned safely to the UK early Saturday morning.
The British government said it was a response to the "most significant violation" of NATO airspace by Russian Preside