For four centuries, Sir Francis Drake has symbolised English nonchalance and cunning in the face of danger. First, according to the legend drummed into every pupil, he insisted on finishing his game of bowls on Plymouth Hoe as the Spanish Armada approached in July 1588. Then he despatched the enemy ships with little more than a few burning rowing boats and a favourable breeze.

But yesterday, it was claimed that Elizabeth's protestant throne was saved by a less celebrated ally: the Turkish navy.

Jerry Brotton, a lecturer at Royal Holloway College, London, told the Guardian Hay literary festival that a hitherto unnoticed letter from Elizabeth's security chief and spymaster, Sir Francis Walsingham, to her ambassador in Istanbul showed that it was Turkish naval manoeuvres rather than Drake's swashbuckling which delivered the fatal blow to the Spanish invasion plans.

The letter, which ordered the ambassador, William Harborne, to incite the Turks to harry the Spanish navy, was written in the mid-1580s and has been buried in archives ever since because it did not apparently relate to any major historical event.

But Mr Brotton told the fes tival: "Walsingham's plan was ultimately successful. Ottoman fleet movements in the eastern Mediterranean fatally split Philip II's armada _ So alongside all the stories we're told at school about why the Spanish Armada failed to conquer Britain and destroy Protestantism, we should add another reason: the Anglo-Ottoman alliance brokered by Elizabeth, Walsingham [and others]."

In his letter to Harborne, Walsingham wrote: "Her Majesty being, upon the success of the said King of Spain's affairs in the Low Countries, now fully resolved to oppose herself against his proceedings in defence of that distressed nation, whereof it is not otherwise likely but hot wars between him and us, wills me again to require you effectually to use all your endeavour and industry in that behalf."

Walsingham hoped that Islamic forces might keep the Spanish forces "thoroughly occupied" by "some incursions from the coast of Africa", or by attacking his Italian territories from the sea.

The Spanish fleet was eventually defeated on July 30 1588 as it awaited the rest of the invasion force off Calais. At the battle of Gravelines, the English navy used fireships before closing in on the confused Spanish.

Hay Festival special, G2