There has been much inconclusive speculation on the Queen’s views on Brexit. In 2016, the Sun asserted that she was in favour (later overruled by Ipso as ‘significantly misleading’). Last year, pro-EU commentators claimed that the blue hat with yellow stars she wore to open Parliament showed coded support for Remain. For now, we are none the wiser. What we do know, however, is that the monarch must be finding things a good deal easier on the way out than on the way in.

Rewind to 1972 and a damp May evening at the Palace of Versailles. The Queen and president Georges Pompidou of France, dressed in their finest, worked their way through a banquet of foie gras, lobster pie, St-Florentin lamb and iced gâteau before rising, on live television, to salute the United Kingdom’s entry to the Common Market. It was the crowning moment of Britain’s accession to the European Economic Community.

Parliament was still angrily debating the issue, and it would be several months before the United Kingdom’s membership formally came into effect. But it was already being treated as a done deal in these high circles. Speaking in French, the Queen proclaimed ‘the beginning of a new Europe, a turning point in its history’. Warm words, but not, it turns out, quite as warm as the hymn of praise her ministers had wanted her to sing.

Here she was in the very city where General de Gaulle had twice declared ‘Non!’ to Britain’s European ambitions. Now, however, his successor was effusive in welcoming the UK. ‘For the first time in more than ten centuries, the peoples of western Europe are definitely committed to follow the path of economic integration and political cooperation,’ declared Pompidou, adding that there could and would be ‘no afterthoughts’.

Forty-four years later, he would be proved wrong. On a night like this, however, Brexit was unthinkable. Le Figaro proclaimed the moment as nothing less than the ‘consecration’ of a new era.

If it was a triumph for both governments, however, it was a very different story for the head of the Commonwealth. Just ask David Cameron. Though his own political career was terminated by Brexit, he acknowledges that, for the Queen herself, Brentry was ‘much harder’.

I have always been struck by the contrasting perceptions of the monarchy here and abroad. Even when the royal family was beset by crises at home, the Queen’s international prestige remained the same. Then, as now, she was seen as a unique and benign bulwark of stability.

Writing my new book, Queen of the World, I have been fortunate to talk to many who have worked closely with her, including six prime ministers (three of them British), six foreign secretaries, all five surviving secretaries-general of the Commonwealth, plus ambassadors and royal staff past and present. I have explored archives, diaries and log books. And what comes through time and again is that this has not been about simply turning up on time. Rather, it’s been a display of that blend of diplomacy, gravitas, charm and statesmanship.

That was particularly true of the Queen’s 1972 state visit to France. Even as negotiations on Britain’s EEC entry were still under way, the Heath government was blithely planning victory celebrations. In October 1971, the Foreign Office asked the British ambassador to France, Sir Christopher Soames, to sound out the French on a triumphal state visit by the Queen. Government papers show that Sir Christopher went directly to President Pompidou, who welcomed the idea ‘avec tout coeur’ and instantly offered a date: May 1972.

The Foreign Office, however, was jumping the gun. There was still passionate anti-EEC feeling at both ends of the British political spectrum. For the Queen, it was personal, too. Her old Commonwealth realms, unflinching allies in two world wars, now faced serious economic problems as Britain turned its back on their markets to join forces with the old enemy. She was their Queen, too. New Zealand was among the worst hit. At the time of the Coronation, two-thirds of its exports were to the UK. Now it was losing its biggest customer. In Australia, deputy prime minister Doug Anthony renounced a lifetime’s ardent loyalty to the Queen and joined the republican movement. Anti-royal sentiment was rising.

The Queen had good domestic reasons for treading carefully. In the very week that Soames was meeting Pompidou, the Commons was in the midst of another furious debate on Britain’s European future. This prompted a stern warning from the Queen’s private secretary, Sir Michael Adeane, that there should be no further dis-cussion of a state visit until the issue was settled. On 28 October 1971, MPs voted by 356 to 244 to move towards membership of the EEC, though there would be months of further toxic debate on the terms of the deal. Yet Edward Heath had no compunction about embroiling the Queen right away. The Queen was off to France while the UK would give the Germans the satisfaction of a state visit to Buckingham Palace soon afterwards.

In Paris, Soames was the ideal ambassador for an event of this magnitude. He had an excellent rapport with the president (whom he had overwhelmed at an embassy luncheon by producing a foie gras dish unique to Pompidou’s corner of the Auvergne). His team of FCO rising stars revered him. One described him as ‘the Great Pachyderm, winning support and affection with tremendous trumpetings, brushing aside opposition with a genial sweep of the trunk and occasionally a savage prod from the tusk’.

All his diplomatic skills were needed when the whole visit was suddenly in jeopardy due to a row over royal transport. The French were adamant that the Queen should travel around France in a French aero-plane. Buckingham Palace was appalled. Her assistant private secretary, Martin Charteris, told Soames that if the Queen were to board a French plane, it would gravely insult the many nations whose planes the Queen had refused to fly in over the years. Only British aircraft would do.

Desperate to satisfy the French, the FCO searched for a precedent. Sure enough, they found that the Queen had once flown in a US presidential plane. The FCO had trumped the Palace. The prime minister formally advised the Queen that she would fly in the presidential Caravelle after all.

Soames had another tricky issue. The Queen wanted to visit her ailing uncle, the Duke of Windsor, at his Paris home during the visit. With the trip fast approaching, the Duke’s health was in rapid decline. A frantic Pompidou asked Soames for an assurance that the Queen would not cancel in the event of her uncle’s death. Soames duly dropped some heavy hints to the Duke’s doctor, Jean Thin. ‘The ambassador told me bluntly that it was all right for the Duke to die before or after the visit,’ Thin told biographer Michael Bloch, ‘but that it would be politically disastrous if he were to expire in the course of it. Was there anything I could do to reassure him?’ There was not.

With days to go, the Palace received top-secret FCO pen portraits of the hosts. Had the French caught sight of these classified documents, Britain’s membership of the EEC could have been torn up there and then. The French prime minister, Jacques Chaban-Delmas, received a vitriolic review: ‘His charm and panache (cultivated almost as a fetish) are offset by his vanity and touchiness… a keenly roving eye.’ Of one minister, Roger Frey, it was observed ‘There is some-thing about the cold blue of his eyes which inevitably recalls the more sinister visions of the Ian Fleming novels.’ Finance minister (and future president) Valéry Giscard d’Estaing was ‘the cactus’ while Pompidou ‘mixes the cunning and mistrust of the Auvergnat countryman with the suavity of the Rothschild director’. As for the first lady, Madame Pompidou was ‘shy with somewhat bohemian tastes’ and ‘living perhaps not very happily in the confines of a gilded cage’.

Finally, on 15 May 1972, the Queen landed in Paris a few hours ahead of that historic address at Versailles. Even as she arrived, Labour’s anti-Common Market cheerleader Peter Shore was publicly attacking her visit as ‘ill-advised’ given that EEC accession was still going through parliament. Foreign Office files give a fascinating glimpse of the squabbles over this most sensitive of speeches. While the Queen still had to maintain her neutrality, the government repeatedly tried to make her voice personal support for the Common Market. One text state d: ‘It gives me great satisfaction to know that the ties between our two countries are being multiplied.’ A stroke of the Charteris pen reduced this to: ‘Every day, the ties between our two countries are being multiplied.’ Another FCO line, ‘I delight that our two countries have found this common sense of purpose’, re-emerged from the Palace as: ‘I hope that our two countries will find a common sense of purpose.’ In other words, they hadn’t found it, and she was not delighted.

Come the night, however, all this was set aside as the Queen (speaking in French) finally heralded this ‘turning point in history’. Summing it up in his despatch to London after the tour, Soames reported: ‘Versailles that evening seemed restored to the purposes for which it had been built, a dream of vanished royal splendour.’

The visit would continue in spectacular style; Soames managed to outclass the French with a British embassy banquet every bit as grand as Pompidou’s (including fillet of beef with ‘pearls of Périgord’ and 1949 Château Latour) plus a white-tie ball for 1,200. The Queen wowed Provence, packed Longchamp, met her dying uncle (who nobly clung to life for nine more days) and left the French enchanted. ‘They saw her as a figure of royal mystique yet with human warmth. It was this that enabled the visit to accomplish, as nothing else could have done, its essential political purpose,’ Soames reported to the foreign secretary. ‘For it was more than merely a glittering and successful piece of international ritual. It was an act of State.’ Britain’s European future was sealed. Or so we thought.

Come the night, all this was put aside as the Queen (speaking in French) heralded this turning point in history

Robert Hardman writes for the Daily Mail. His book, Queen of the World, is published by Century at £25.