Sixteen crates locked in a dark store room in Madrid for more than 70 years hold the secret to how General Franco might have won the Spanish Civil War.

Inside the crates are Enigma code-making machines that Franco had bought from Nazi Germany and used to co-ordinate his troops who fought on fronts hundreds of miles apart.

The 26 machines were discovered this week by the Spanish daily newspaper El País, hidden in army headquarters since the Civil War ended in 1939, most still in perfect condition.

The Enigma machines gave Franco's Nationalists a crucial advantage because their code was never cracked by their Republican foes. Hitler used the machine to devastating effect to command his forces during the Second World War, until the code was finally deciphered by cryptologists at Bletchley Park, Oxfordshire.

After the Nationalist rebellion in July 1936, General Franco realised that the Civil War could be won only by co-ordinating simultaneous offensives in different parts of the country.

Franco needed to improve communications between his generals as encryption of top-secret messages was far from sophisticated: at the start of the war both sides used the same codes.

He bought ten Enigma machines from Germany, distributing them among his generals. Hitler supported Franco, but only to a degree.

The Madrid cache comprises commercial Enigma machines, invented in 1920, rather than the more effective and highly secret military version that the German army used so effectively during the war.

Rafael Moreno Izquierido, a professor of journalism at Complutense University, Madrid, said: “Hitler was aware of the risk of one of these machines falling into British or Soviet hands and most likely did not trust the Spanish fully as the smallest slip-up could have compromised his secret weapon.”

Even the inferior Enigma machine was highly complex and required two operators, with each message assigned a different code.

Commander Antonio Sarmiento was in charge of training operators at Franco's headquarters in Salamanca.

In a 1936 report he said: “To give an idea of how secure these machines are, suffice to say that the number of possible combinations is a remarkable 1,252,962,387,456.”

Franco was said to have taken an Enigma machine with him when he travelled to the front. Impressed with their performance, he eventually bought up to 50 of them, and they were in use until the 1950s.

Professor Denis Smyth, of the University of Toronto, an expert on Second World War intelligence operations, said that the British code breaker Alfred Dilwyn Knox cracked the code of Franco's machine in 1937, but “this information was not passed on to the Republicans”.

Some of the surviving machines are displayed in Spanish military museums. Those unearthed by El País were found by chance in an attic of the Army headquarters when an inventory was being carried out.

British efforts to crack the Enigma code used by Germany were dramatised in the 2001 film Enigma, starring Kate Winslet.

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