LONDON — Mon dieu! The British queen as la reine de la France? C’est impossible, n’est-ce pas? (Translation: “My God! The Queen as the queen of France? It’s impossible, isn’t it?”) Not entirely, according to documents housed at Britain’s National Archives in Kew.

They show that in 1956 Britain and France considered a union and the possibility of the British queen as the first regal head of this avowedly republican country since previous occupants of the post literally lost theirs.

The prospect rendered one eminent French historian almost speechless yesterday. “Really, I am stuttering because this idea is so preposterous,” stammered Henri Soutou of the Sorbonne.

But a British cabinet note shows that the French prime minister, Guy Mollet, first broached the subject during a meeting in London with his British counterpart, Anthony Eden.

The letter from September 10, 1956, states: “When the French prime minister, Monsieur Mollet, was recently in London, he raised with the prime minister the possibility of a union between the United Kingdom and France.”

When that idea was rejected, an undaunted Mr. Mollet had another shot when Eden visited Paris a fortnight later. What about the French joining the commonwealth, he suggested?

He even ventured that there would be “no difficulty over France accepting the headship of Her Majesty,” according to a note on September 28 of a conversation between Eden and his cabinet secretary, Sir Norman Brook. Tempting though it was, that idea, although greeted with more enthusiasm, was eventually rejected, too.

Eden was persuaded by his cabinet that Belgium, Holland, West Germany, and Italy might all want to follow suit, which was complicated. As history shows, a year later, France signed the Treaty of Rome with Germany and Elizabeth II paid a state visit to France.

As flag-waving Parisians turned out to give her a warm welcome, none was aware that they were looking at the woman who could have been the first French Queen since “Madame Guillotine” dispatched the last.

Yet such a move was not perhaps as outrageous as it first seems. France was facing severe economic difficulties and also an escalating Suez crisis.

It wanted the British on side to help retake the canal from Egypt, which was funding separatists in rebellious French Algeria.

There was also tension on the Israel-Jordan border. France was an ally of Israel and Britain of Jordan. If that erupted, French and British soldiers could end up fighting each other.

“The idea of joining the commonwealth and accepting the headship of Her Majesty would not have gone down well,” Mr. Soutou told BBC Radio 4’s Document program, which broadcast “A Marriage Cordial” on the subject yesterday. “If this had been suggested more recently, Mollet might have found himself in court.”

Denis MacShane, the Francophile former Europe minister, said it was a fine example of the “tortured romance” between the nations that had existed “since William the Conqueror colonized Britain 1,000 years ago.”

“Churchill offered to the French to merge completely with Britain in 1940, which the French turned down. Guy Mollet was a teacher of English from Calais. I suspect he was seeking to copycat that as France was under terrible pressure.”