In May 1940, captured British soldiers from the Royal Warwickshire and Cheshire regiments were herded into a 20ft by 10ft barn, bombarded with grenades, then machine-gunned. Only a dozen survived, of whom four are still living.
Wilhelm Mohnke, 82, who lives near Hamburg, has been identified by witnesses, and named in Parliament as the officer responsible for ordering the massacre at Wormhoudt, northern France, and at least two similar atrocities.
The decision to release the documents, which were due to remain closed for at least another 12 years, came yesterday in a letter from the Secretary of State for Defence, Malcolm Rifkind, to the all-party Parliamentary War Crimes Group.
It is almost certainly an attempt to pre-empt an imminent announcement by the German authorities that they have decided not to prosecute Mr Mohnke. Britain has no jurisdiction over the incident because it took place in France.
The German Ministry of Justice, which has been studying the British files for four years, told the Independent yesterday it would be disclosing the results of its investigation into Mr Mohnke's past within the next three weeks.
The 22 secret files, belonging to the Ministry of Defence, were compiled by Allied war crimes investigators in 1945 and 1946. Mr Mohnke was not considered for prosecution at the Nuremburg trials because he was captured by the Soviet Union at the end of the war. By the time he was released in the mid-1950s, his case and hundreds of others left on the files had effectively been forgotten by British authorities. He returned home and enjoyed a successful business career.
General Mohnke was a founder-member of Hitler's SS bodyguard, the Liebstandarte, in 1933, and marched at the head of the Nazis' victory parade through Berlin after the fall of France.
He commanded an SS regiment and stayed with Hitler until his final hours, when he advised him the end had come and set fire to the bunker after his leader's suicide.
The British documents, which will be released at the Public Record Office early in the New Year, are army files whose findings have never been revealed. The files have been among the evidence studied for the past four years by the German authorities.
General Mohnke, probably the last surviving close confidante of Hitler, is also allegedly implicated in a second massacre at Malmedy, in the Ardennes, of 72 American prisoners of war in 1944, and a third incident the same year when three Canadians were killed.
Mr Rifkind makes it clear in a letter to Lord Merlyn-Rees, chairman of the all-party Parliamentary War Crimes Group, that having co-operated with the German inquiry and waited four years, the British want the evidence in the open as soon as possible.
His letter implies that only the workload from the annual release of documents at the Public Record Office every January prevents their publication this month.