It's long been claimed that Winston Churchill temporarily gave a London hotel room to Yugoslavia so a prince could be born on Yugoslav territory. But finding evidence to support the story is hard, and perhaps impossible.
On 17 July 1945, the United Kingdom became a little smaller, and Yugoslavia a little bigger.
On the orders of Winston Churchill, suite 212 of Claridge's Hotel in London became Yugoslav territory, for one day only.
This allowed Crown Prince Alexander of Yugoslavia - part of the exiled royal family - to be born on home territory.
The front door became an international border. Room service was made in one country, and delivered in another.
Just one problem. There is - it seems - no proof that it's actually true.
In April 1941, during World War Two, Germany and its allies attacked and occupied Yugoslavia, so King Peter II of Yugoslavia - then just 17 years old - left the country with his government, and travelled to London via Athens, Jerusalem and Cairo.
King Peter and his wife, Queen Alexandra, took up residence in one of London's most fashionable hotels, Claridge's, and it was here four years later that the queen became pregnant.
This presented the royal couple with a dilemma.
They wanted their first-born - and, in their eyes, the future king or queen of Yugoslavia - to be born in the country he or she would rule. But, with the war in Europe over, Yugoslavia was on its way to becoming a communist republic.
And so, the story goes, British Prime Minister Winston Churchill made King Peter II an offer to redraw national boundaries - to make a small part of London Yugoslavia, allowing the future king to be born on home turf.
Had this offer really been made, one might expect some evidence to exist. An act of parliament, perhaps, or at least an official memo. Even a newspaper story, telling the British people their country had shrunk.
But it seems there is nothing.
The Churchill Archives - which contain millions of digitised documents - can find no evidence.
The National Archives has also found no trace. Indeed, the only document from 1945 in the archives that mentions Yugoslavia and Claridge's is a letter to the British Foreign Office, dated 23 July 1945.
Signed Dr D Protitch, counsellor, Yugoslav Embassy, it states: "I now have the honour to inform you that Her Majesty Queen Alexandra gave birth to a son at Claridge's Hotel, and that Her Majesty's identity card and ration book bear the address of 41 Upper Grosvenor Street, W1."
There is no mention in the letter of Churchill's deal with King Peter II.
There's also no mention of it in Hansard, which records every word spoken in the British parliament, or - it seems - in any British newspaper.
The deal is absent from Sir Martin Gilbert's biography of Churchill, and from the diaries of Sir Alan Lascelles, the private secretary to Britain's King George VI. (In some versions of the story, suite 212 was ceded by King George VI, who then became Crown Prince Alexander's godfather.)
And so - if there's no proof of Churchill's deal in Britain - does it exist elsewhere?
Crown Prince Alexander
- Recognised in his homeland as Crown Prince of Yugoslavia from his birth on 17 July 1945 until the abolition of the monarchy in November the same year
- Christened in Westminster Abbey - George VI was one godparent, his daughter, the Queen, was the other
- Went to school at Gordonstoun in Moray - Prince Charles's school - and Millfield in Somerset, among other places
- Attended the Royal Military Academy at Sandhurst, and rose to the rank of captain with tours in the Middle East, Northern Ireland and West Germany, before he left the Army in 1972
- Head of the House of Karadjordjevic and claimant to the throne of the defunct Kingdom of Serbia
"Unfortunately, all the files have long ago disappeared from my father's office," Crown Prince Alexander told the BBC.
"The files included all the wartime ones - especially the large correspondence of letters from Churchill, Roosevelt, Stalin, and many more.
"The document regarding the temporary secession of the room relating to my birth also went missing.
"When I was born, I never got a British birth certificate, since I was technically born in Yugoslavia."
Crown Prince Alexander's office says Churchill's offer was "most likely" made in May 1945, during a meeting with King Peter II.
So can prime ministers give away parts of their country without passing a law?
"There is no power of which I am aware for the prime minister, or anyone else, to designate any UK territory as even temporarily someone else's - it would require an act of parliament," says Dr Bob Morris, a constitutional expert at University College London, who spent almost 40 years as a civil servant at the Home Office.
"Somehow or other a romantic fiction took root with an overlay of imagined credibility based on Churchill's reputation for impulsive generosity."
What is certain is that the young Crown Prince received a Royal Yugoslav passport, rather than a British one.
But using it to travel - as Marshal Tito's communists governed Yugoslavia - was "very difficult and eventually impossible", according to Crown Prince Alexander's office.
Having abolished the monarchy in late 1945, Tito's government stripped the royal family of their Yugoslav citizenship on 8 March, 1947, confiscating their property in the same decree.
In Crown Prince Alexander's teens, an 18th Century law was used to make him a British subject.
The Sophia Naturalization Act of 1705 was used to make Sophia - the granddaughter of James I, and the mother of George I - an Englishwoman.
It also allowed her descendants to become English subjects. One of them - her great, great, great, great, great, great, great, great grandson - is Crown Prince Alexander.
As a British citizen, the Crown Prince became an officer in the British Army, and later worked in finance in the United States. Meanwhile the fall of communism in Yugoslavia led to the Balkans Wars and the break-up of the country.
But after Slobodan Milosevic left office in 2000, the new government of Yugoslavia (by now consisting of only Serbia and Montenegro) reached out to the royal family and reinstated their Yugoslav citizenship. The ceremony took place on 12 March 2001, in - where else? - suite 212 of Claridge's.
"I started my life as an emigre," Crown Prince Alexander said at the time. "I'm very happy that as a family we can return home as citizens."
Just over four months later, the family moved into the royal palace in Belgrade. The date was 17 July 2001.
Exactly 56 years after being born in London - in a hotel room that may, or may not, have been Yugoslav territory - Crown Prince Alexander was back in his homeland.