In September 1936, Prince Kiril accompanied King Edward VIII on a whistle-stop tour of Iceland.
Present at the death of his brother, Tsar Boris, on 28 August 1943, Prince Kiril was appointed head of a regency council by the Bulgarian parliament, to act as Head of State until the late Tsar's son, Simeon II of Bulgaria, became 18.
Prince Kiril, with the widowed Tsaritsa, Giovanna of Savoy, daughter of the Italian king, led the state funeral for his brother Tsar Boris III on 5 September 1943 at the Alexander Nevsky Cathedral, Sofia, thereafter proceeding across the city to the main railway station where the funeral train waited to take the body to the 12th-century Rila Monastery in the mountains. Thereafter, three consecutive governments made efforts to extricate themselves from Bulgaria's agreements with Germany, notably that which permitted their use of the railway to Greece and the German troops stationed along it for protection. A Bulgarian delegation travelled to Cairo in an attempt to negotiate with the United States and the United Kingdom but failed, as the latter refused to meet the delegation without the participation of the Soviet Union.
Despite Sofia's continuous diplomatic ties with the Soviet Union, on 5 September 1944, that country declared war on Bulgaria, and on 8 September Soviet armies crossed the Romanian border and the Danube. The Fatherland Front, a coalition of the Communist Party, the left wing of the Agrarian Union, the Zveno group, and a few pro-Soviet politicians who had returned from exile in the Soviet Union, executed a Soviet-backed military coup on 9 September and seized power.
In late January 1945 Prince Kiril was sentenced to death by the People's Court. On the night of February 1, 1945 he was executed at Sofia Central Cemetery along with former Prime Minister and Regent Professor Bogdan Filov, Regent General Nikola Mihov, and a range of former cabinet ministers, royal advisors and 67 MPs.
On August 26, 1996, the Supreme Court overturned the sentences of February 1, 1945, which had sentenced the three regents, ministers, and councilors to death.
