The Western Front during the Battle of Somme
German prisoners captured in the run up to the bloodbath in northern France gave the British detailed information on how to beat their enemy - but they were ignored.
Captives gave the British top brass reports of where a massive week-long bombardment had destroyed the German defences and where the shells had been ineffective, according to a new book on the historic battle.
British troops were told sections of the German's southern defences were badly damaged by the battle, but elsewhere bunkers were deeper and likely to be more resilient.
But the intelligence was ignored or misunderstood, resulting in British commanders assuming the German defences were damaged across their whole length and pushing ahead with plans to attack on all fronts.
The Battle of Somme between the British and French alliance and the Germans over five months was one of the fiercest battles of World War One.
In total more than one million soldiers were killed and injured on both sides.
A tomb of the Unknown Soldier in Westminster Abbey
It was completely unnecessary, they had clear intelligence
Around 310,000 soldiers died, with nearly 20,000 British soldiers slain on the first day alone.
Details of the intelligence failings were found in prisoner resorts unearthed by author Hugh Sebag-Montefiore for a new book on the battle, called Somme: Into the Breach.
German deserters who stumbled into British trenches from the German frontline at Montauban told their interrogators the frontline where they had stood guard was just "a line of shell holes" and most of the dugouts had "collapsed".
But the captives also suggested there were far deeper and stronger dugouts elsewhere in the line which were far more likely to resist artillery.
Gas-masked men of the British Machine Gun Corps
The reports show British interrogators believed the deserters were telling them the truth.
Mr Sebag-Montefiore said: “They knew that there was a weak point in the German line and that the rest of the line was likely to have held. Yet they attacked in the same strength all long the line.
“All those lives lost at the north end of the line, it was like a massacre. It was completely unnecessary, they had clear intelligence.
Trenches at the Battle of Somme
“If they had really just concentrated on the southern part of the line, they might really have broken through.”
Despite the prisoners' information, an intelligence briefing told commanders immediately before the battle that “everything was wide open. They thought they could walk through everywhere”.
The reports were either deliberately buried or ignored - despite turning out to be correct.