War diaries, witness statements and documents have revealed that 21 soldiers from the Royal Scots were massacred after surrendering to German SS troops in May 1940.

The men were part of a desperate rearguard trying to hold back the advancing Germans to buy time for the evacuation of the main British army from the beaches in and around Dunkirk.

It had previously been believed that 97 members of the Royal Norfolk Regiment were the only victims of the SS Totenkopf Division in a known atrocity near the village of Le Paradis in northern France.

Now Hugh Sebag-Montefiore, the author of Dunkirk - Fight to the Last Man, has uncovered evidence that local French civilians later dug up a mass grave containing the bodies of Scottish troops.

All showed signs of having been shot, execution style, through the back of the head or neck.

The information was contained in a Norfolks' officer's war diary that had lain unread for more than 60 years.

It also corroborated a statement taken from a German dispatch rider after the war that he had been told by a comrade that 17 British soldiers were captured hiding in a hay loft in the area occupied by the Royal Scots.

The prisoners had been taken to the SS battle headquarters, where the witness stated that "they all had to bite the dust".

A report by another German soldier, found in archives after the war, reported the summary execution of a lone British soldier near Le Cornet Malo, a village in the Royal Scots' sector close to Le Paradis.

This man's only crime was that he had sniped at SS soldiers from a house and had then been shot in revenge for his lethal marksmanship.

The contents of the new documents are given further credibility by the account of Sergeant-Major Johnstone, a Royal Scots NCO interviewed by war crimes investigators seeking evidence on the Le Paradis massacre.

Johnstone testified that he and a group of his soldiers had been lined up by a roadside ditch after they surrendered and were disarmed and that SS troopers were preparing to machine-gun them.

The party was reprieved at the last minute when a passing German staff officer intervened and ordered the men be treated as prisoners-of-war.

The officer then congratulated the Scots on staging such a courageous last stand. "You fought like tigers," they were told before being marched off to captivity.

The Royal Scots were part of 4 Brigade, one of the rearguard units that fought the Germans to a standstill on the Dunkirk perimeter.

Unknown at the time to the soldiers battling German tanks with little more than rifles and machine-guns, their sacrifice allowed more than 300,000 men to be taken off the beaches behind them by an armada of warships and small boats.

The slaughter of the Norfolks was thoroughly investigated not only by allied authorities but also by the regular German army.

A German major assigned by the Wehrmacht's 16th Corps to look into allegations of an atrocity reported that he had seen the bodies of 89 British soldiers who had been shot in the head at close range or bludgeoned to death by rifle butts.

Despite probing questions on the conduct of the SS, his report was shelved when the Totenkopf Division was transferred from army control.

Documents from the Totenkopf (Death's Head) Division headquarters reveal that senior officers were aware of what had taken place, but dismissed the killings as a response to British troops using outlawed dum-dum bullets doctored to inflict horrendous wounds. No evidence was presented to support this claim.

General Theodor Eicke, the divisional commander, also claimed that some of his own men had been shot in the back and insisted that the British soldiers had been executed after a field court-martial proved their "villainous tactics".

Mr Sebag-Montefiore said yesterday: "I would very much like to hear from former Royal Scots or their descendants who have any information on the incidents near La Paradis.

"This was a terrible atrocity carried out in cold blood and should be documented accurately for posterity."

The Totenkopf Division lost more than 700 men in the battle for France and was virtually wiped out on the eastern front against the Soviet Red Army later in the war.

The Royal Scots, now amalgamated into the Royal Scots Borderers, recruited in Edinburgh and the Lothians.

The regiment, nicknamed "Pontius Pilate's Bodyguard" due to its seniority in the British Army and position as First of the Line, can trace its history back more than 300 years.

Mr Sebag-Montefiore can be contacted at (020)8 444 1684 or by e-mail at sebags@hsmontefiore.com.