The Romanian Royal Family has issued a statement announcing the death of King Michael I, aged 96, on 5 December at his home in Switzerland.
The former king was diagnosed with cancer last year.
President Klaus Johannis announced on Tuesday that a day of mourning to mark the former monarch’s passing.
His body will lie in state at Peles Castle in the Carpathian mountains and later in Bucharest before his burial in Curtea de Arges.
King Michael, or Mihai, reigned from 20 July 1927 until 8 June 1930, then from 6 September 1940 until his abdication on 30 December 1947.
He was one of the last surviving World War II leaders.
He had been ill for some time and unable to attend his wife Queen Anne’s funeral in Romania last year.
The only son of Carol II and his second wife Princess Helen of Greece and Denmark, Michael was born on 25 October 1921 at Peles Castle in Sinaia in the Carpathian mountains. He was a descendant of the German Hohenzollern dynasty and one of Queen Elizabeth II’s many cousins through their ancestors Queen Victoria and Christian IX of Denmark.
However, Carol was forced to renounce his rights to the throne in 1925, following his scandalous affair with courtesan Eleni ‘Magda’ Lupescue shortly after Michael was born. Carol and Lupescu lived in exile after moving to Paris.
Meanwhile, Michael’s grandfather Ferdinand I died on 20 July 1927 so he became king when he was only five years old. However, as he was still a minor a regency council was established which included his uncle, Prince Nicholas.
However, the council was so ineffective that Carol returned to Romania and replaced his son, reigning as Carol II. Michael, once again, was the heir apparent.
Carol II was deposed in 1940 and Michael became king again. Michael played a major part by changing sides during World War II, during his 20s, switching from an alliance with the Nazis, after participating in a coup in 1944 where fascist leader Marshal Ion Antonescu was overthrown, before joining the Allies.
He was forced to abdicate in 1947 after a Communist takeover, stripped of his citizenship and forced to live in exile for decades, mostly in Switzerland, Britain and the United States.
He married Princess Anne of Bourbon-Parma in Athens, Greece, on 10 June 1948 after meeting her at his cousins’ wedding, the future Queen Elizabeth II and Prince Philip Duke of Edinburgh, on 20 November 1947.
They had five daughters: Crown Princess Margareta (born 26 March 1949), Princess Elena (born 15 November 1950), Princess Irina (born 28 February 1953), Princess Sophie (29 October 1957) and Princess Maria (born 13 July 1964).
Michael and Anne have five grandchildren and three great-grandchildren.
Michael worked as a poultry farmer, a test pilot, entrepreneur and stockbroker whilst maintaining links with the national committee he had established to maintain links with his homeland.
Communist dictator Nicolae Ceaușescu was overthrown and executed in 1989 but the government, fearing a surge of support for King Michael, refused to grant entry on his first few attempts, including sending tanks to stop him from touring Romania during the early 1990s.
However, Michael was finally allowed to return to Romania in 1992 and regained citizenship in 1997.
He recently removed his grandson Nicholas Medforth-Mills from the line of succession and stripped him of his titles, HRH and Prince of Romania.
His wife, Queen Anne passed away in 2016 with the Romanian government declaring a national day of mourning on 13 August.
At 96, he was the second-oldest former monarch, after Jean, Grand Duke of Luxembourg, and four months older than consort Prince Philip Duke of Edinburgh.