The reasons behind Sir Anthony Eden's mistake in dispatching British troops to Suez are among the most enduring mysteries of modern politics.
Rumours have circulated for decades that Eden, who was Prime Minister from 1955 to 1957 and was suffering from a debilitating illness at the height of the crisis in 1956, was taking addictive, painkilling drugs that could have clouded his judgement.
Private papers just uncovered in the Eden family archives provide a definitive answer, disclosing that he had been prescribed a powerful combination of amphetamines and barbiturates called drinamyl.
Better known in post-war Britain as "purple hearts", they can impair judgement, cause paranoia and even make the person taking them lose contact with reality. Drinamyl was banned in 1978.
The document was obtained by David Owen, the former foreign secretary, as he researched a book on the effects of ill-health on key moments in history. Written by Eden's physician, the document confirms the Tory prime minister had been prescribed drinamyl.
Lord Owen will reveal his discovery tomorrow, the 50th anniversary of British troops arriving in Egypt on their ill-fated mission to seize the Suez Canal.
"There have been rumours and suppositions that Eden had been taking these drugs over many years," Lord Owen told the GMTV Sunday Programme. "But no one has been able ever to prove it and some have denied he was even taking these drugs. Everybody knows that when you are taking these drugs and are having a down period or difficulty, it's extremely frequent for patients to up the dose themselves and not tell those closest to them or even their doctors."
Eden's medical woes dated back to 1953 when a surgeon's hand slipped during a routine gall bladder operation, severing Eden's bile duct.
The mistake led to years of cholangitis, an abdominal infection which became so agonising three years later that he was admitted to hospital with his temperature reaching 106F.
By this time he had succeeded Sir Winston Churchill and the Suez crisis was coming to a head after the canal was seized by President Nasser of Egypt.
While taking the painkillers, Eden, who was widely believed to be obsessed with the threat from Nasser, secretly formulated a plan with French officials under which Israel would invade Egypt, allowing British and French forces to intervene and seize the canal.
The scheme backfired spectacularly. The military action split the Cabinet, divided public opinion, undermined the economy and was condemned by world leaders, including US President Dwight Eisenhower.
A humiliated Eden was forced to declare a ceasefire and, with his health continuing to decline, he resigned as Prime Minister in the following January.
The Suez debacle, judged by many as marking the end for Britain as a major power in the world, has ever since haunted politicians who saw how it wrecked Eden's reputation.
Lord Owen, who qualified as a doctor before pursuing his political career, said: "Eden would not have contemplated such an action if he had been fit and well."
Malcolm Lader, a professor of clinical pharmacology at King's College, London, said people taking drinamyl become "disinhibited" and start acting out of character. In larger doses, he said, they can become paranoid and their judgement "becomes even more impaired - at the most extreme they can lose contact with reality".
Lord Owen also accused Tony Blair of not telling the full truth about his erratic heartbeat, which was eventually cured with surgery. He said the Prime Minister "pretended" shortly after the invasion of Iraq in 2003 that he had not previously suffered heart problems.
His assertion was later contradicted by former US president Bill Clinton and by David Blunkett, the former home secretary.
Lord Owen said the public was entitled to know the full truth about any prime minister's health, particularly in times of crisis.
"I'm not saying this influenced the way he made his decisions, but we have no idea whether he was on any treatment," Lord Owen said.