"I welcome the fact that the Crimean Tatar leaders Ilmi Umerov and Akhtem Chiygoz were freed on October 25," the FCO quoted the minister as saying in a statement posted on its website on October 26. "They were both arrested and sentenced by Russia for their opposition to the illegal annexation of Crimea."

He said the UK had previously called for their release, and appreciates the efforts of all those who worked for this outcome.

Read alsoCrimean Tatar Mejlis members freed by Russia expected in Kyiv on Oct 27"Nevertheless we must not lose sight of the many others, including other Crimean Tatars, who have been detained by the de-facto Russian authorities for their opposition to the illegal annexation of Crimea. I call once again for the immediate release of all Ukrainian political prisoners on the Crimean Peninsula and in Russia," he said.

"The UK remains clear that Crimea is part of Ukraine, and that its annexation by Russia is a breach of international law," he added.