The letter was sent to President of the European Council Donald Tusk, President of the European Parliament Antonio Tajani MEP, President of the European Commission Jean-Claude Juncker and High Representative of the European Union for Foreign Affairs and Security Policy Federica Mogherini.

In it, the signatories write that they are "deeply concerned about the critical condition of the political prisoner Oleg Sentsov, who has been on hunger strike since 14 May 2018."

They demand his immediate release, without any preconditions added such as a personal request for pardon to Russian President Vladimir Putin. "We also demand immediate release of all other Ukrainian nationals who are currently being held in prisons in Russia under politically motivated charges," they write.

Writer and filmmaker Oleg Sentsov has been incarcerated in Russia since August 2015. He was tried as a terrorist and sentenced to 20 years in prison. Although Sentsov never gave up Ukrainian citizenship, the Russian authorities declared him a Russian citizen, which they have since used as grounds not to extradite Sentsov to Ukraine.

The Russian authorities' actions against Sentsov are politically motivated and happening "because of his critical attitude towards Russia's aggression against Ukraine and, first of all, towards Russia's occupation of Crimea," the MPs write.

"Oleg Sentsov announced his hunger strike in protest against injustice, in order to draw the attention of the international community to the situation of political prisoners jailed or detained in Russia and in Crimea, occupied by Russia, and demand their release. Oleg Sentsov has reached 90 days of hunger, and his condition is being described as catastrophic. In addition, he is denied correspondence, meetings with independent observers and adequate medical care. There are reports about him being continuously bullied. Unless rapid action by the international community is taken, the democratic and free world will have another case similar to that of Sergey Magnitsky on its hands," the letter reads.

"We call on the leaders of the European Union to step up pressure on the Russian authorities and President Putin demanding immediate release of Oleg Sentsov and other Ukrainian nationals held under politically motivated charges in Russia's prisons."

"We call on the European Union to identify persons and officials who have issued orders, executed, facilitated and were otherwise involved in detaining, convicting and imprisoning Oleg Sentsov and announce personal targeted sanctions against them as perpetrators of gross human rights violations," the signatories wrote.

The appeal was signed by the following current and former members of the European Parliament, the Riigikogu (Estonia), the Saeima (Latvia) and the Seimas (Lithiania):

• Gabrielius Landsbergis, Member of the Seimas, Chairman of the Homeland Union - Lithuanian Christian Democrats, Lithuania
• Radvilė Morkunaite-Mikuleniene, Member of the Seimas, Lithuania
• Andrius Kubilius, Member of the Seimas, Lithuania
• Tarmo Kruusimäe, Member of the Riigikogu, Estonia
• Tunne Kelam, Member of the European Parliament, Estonia
• Mari-Ann Kelam, former Member of the Riigikogu, Estonia
• Ojārs Ēriks Kalniņš, Member of the Saeima, Chairman of the Foreign Affairs Committee, Latvia
• Veiko Spolītis, Member of the Saeima, Latvia
• Atis Lejiņš, Member of the Saeima, Latvia
• Rasa Jukneviciene, Member of the Seimas, President of the NATO Parliamentary Assembly, Lithuania
• Prof. Inese Vaidere, Member of the European Parliament, Latvia
• Krišjānis Kariņš, Member of the European Parliament, Latvia
• Liisa Oviir, Member of the Riigikogu, Estonia
• Enn Meri, Member of the Riigikogu, Estonia
• Henn Põlluaas, Member of the Riigikogu, Estonia
• Mart Nutt, Member of the Riigikogu, Estonia
• Sandra Kalniete, Member of the European Parliament, Latvia
• Jürgen Ligi, Member of the Riigikogu, Estonia