How Harold Macmillan and I saved a Leonardo for the nation

If the National Gallery's most ambitious hopes are fulfilled, its imminent exhibition on Leonardo da Vinci will prove the mightiest blockbuster ever staged there. For several months, the gallery has been urging visitors to buy advance tickets, "to ensure that they can view the exhibition and avoid disappointment". Public demand, quickened by the global success of Dan Brown's novel The Da Vinci Code, is already intense.

Leonardo has the ability to ignite enormous fascination, even among people who know nothing about art. And London itself has become one of the world's most successful cities at attracting viewers to museums and galleries. The Leonardo show, giving us a once-in-a-lifetime chance to see so many of his paintings and drawings in one place, will be the talk of the town. Running for nearly four months and staying open until 10pm on Fridays and Saturdays, it is bound to outstrip the most heady expectations of its organisers.

London's current reputation as a blockbuster capital compares dramatically with the city I discovered as a youth. Avid for art from an early age, I drew incessantly and hitch-hiked from Kingswood, my boarding school in Bath, to roam around London's rich collections. But in the early 1960s, Leonardo was difficult to track down. The outstanding array of his drawings in the Royal Collection was tucked away at Windsor Castle. As for the National Gallery, it owned only one picture securely attached to his name: the second version of The Virgin of the Rocks, painted in Milan around 1508. Now, in the new exhibition, the Louvre is lending to London its first version of The Virgin of the Rocks. So these two major images will be united for the first time, something that is bound to encourage enormous debate.

When I encountered Leonardo's painting as an awed schoolboy, the National Gallery was a hushed and under-populated location where young people did not feel at home. But I fell in love with The Virgin of the Rocks. In 1961, I spent six shillings on a Pelican paperback of Kenneth Clark's pioneering book on Leonardo. Looking at its yellowing pages now, I realise that Clark must have alerted me to the existence of what he called the "Burlington House cartoon". Praising it unreservedly as "one of Leonardo's most beautiful works", Clark made me want to see this large and irresistible drawing, executed around 1507-08 and preserved since the 18th century in the collection of the Royal Academy.

Poring over photographs of the cartoon, which shows the Virgin and Child with St Anne and St John the Baptist, I understood at once why Leonardo's draughtsmanship had, according to his first biographer Vasari, "filled all artists with wonder".

Suddenly, before I had a chance to track down the cartoon, the Royal Academy announced on 10 March 1962 that the drawing would be sold at auction. Sir Charles Wheeler, the Royal Academy's president, had very reluctantly decided that a rapidly increasing bank overdraft would be halted only by this drastic act of sacrifice. Horrified by the news, I sat down and wrote an indignant letter to The Daily Telegraph, which had been delivered to our family home throughout my childhood. I was only 14, and could not imagine that my missive would be taken at all seriously by the newspaper. Yet I still felt that it must be written.

To my amazement and delight, the Telegraph lost no time in publishing my letter, under the headline "High Price to Pay for Independence", on 14 March. "I would like to point out what seems to me a flaw in the Royal Academy's reasons for the sale of Leonardo's magnificent cartoon," I wrote. "If the picture goes to the highest bidder at some astronomical price, the buyer will almost certainly be an American. Thus, if an export licence is, as I hope, refused, the Government will no doubt have to bear the main burden of financing the National Gallery to purchase the picture. Surely it would be much easier for the Government merely to finance directly the Academy. The argument for the Academy is, I gather, that they want to remain an independent body, but surely to risk the export of one of the greatest pictures in England seems a heavy risk to run for the sake of the tradition, however great, of the Royal Academy."

It was the first time anything I had written about art had been published. The frisson on opening a newspaper and finding my own words printed inside cannot be exaggerated. And very soon afterwards, I felt even more gratified to learn that a formal meeting had been held on 16 March at the Treasury, which agreed to support and assist a public appeal to buy the imperilled cartoon.

This crucial meeting was brought into being by the directors of the British Museum and the National Gallery, both anxious to save Leonardo's sublime drawing for the nation. Several American galleries were determined to acquire it, and one of them had £2.5m available for the purchase. It was an unbelievable amount, at a time when no work of art had ever fetched £1m at auction.

Day by day, the sense of urgency grew stronger. A second meeting decided that the appeal was more likely to succeed if it announced that the National Gallery would be the cartoon's permanent home. The National Art Collections Fund, under the energetic leadership of its chairman Lord Crawford, took on the task of organising the campaign. And on 30 March, just over a fortnight after my letter had been published, the appeal was launched with a price fixed at £800,000 – still widely regarded as a colossal sum for any artwork, let alone a fragile drawing executed on eight sheets of paper glued together.

I was among the 380,000 people who thronged to see the cartoon during the first month of its public display. It was a landmark moment in the history of the blockbuster exhibition in Britain. The response was so positive that public donations began pouring in from individuals and more than 1,000 schools all over the country. One deranged visitor defied the National Gallery's elaborate security precautions and hurled a bottle of ink at the cartoon, penetrating its perspex shield. But no major damage was caused, and by the end of July more than half the price had been raised.

Looking at the cartoon, I warmed to Leonardo's free, immediate handling of charcoal heightened with white chalk. While giving the group of four figures a sculptural power, he emphasised at the same time the spontaneous human affection between them – above all where the Christ Child leans across to bless the infant Baptist and touch him playfully under the chin. St Anne, the Virgin's mother, smiles at her daughter with an intense devotion, and points up to heaven. But Leonardo reduces the whole of her hand to a minimal outline. Its emptiness makes us aware of just how ghostly the other figures are as well.

Even as Leonardo celebrates life and love, he stresses the vulnerability of adults and children alike. This sense of transience and mortality meant a great deal to me, since my own father had been wounded by a Nazi machine-gunner and captured soon after D-Day. So I was overjoyed when Harold Macmillan, the Prime Minister, finally announced on 31 July that a special government grant of £350,000 would rescue Leonard's work for the nation.

Seeing it on display in the National Gallery's new exhibition, I will undoubtedly feel delighted that my teenage letter was able to play a modest part in ensuring that Leonardo's redemptive vision can now be savoured by us all.

Leonardo da Vinci: Painter at the Court of Milan, National Gallery, WC2N, from Wednesday