On June 9, 1942, a speech was broadcast on the BBC that would change human history.

Władysław Sikorski, Poland's prime minister exiled in London, revealed that 700,000 Jews had been systematically murdered in brutal Nazi concentration camps, quarantined and executed en-masse in ghettos, and walked to their deaths in gas chambers.

This was the first time the world and the Allied forces had heard of the crimes of the Holocaust, a secret the Nazi leaders had been able to keep until now.

The information, a dossier of photographs and documents, had come to London from Poland via Stockholm, thanks to a group of Swedish men who had risked their lives to tell the world about the Nazi persecution of Polish Jews

Forgotten hero: Sven Norrman, responsible for couriering the documents that would reveal the Nazi Holocaust and slaughter of 700,000 Polish Jews out of Warsaw, pictured with his Polish-Jewish mistress Gizela 'Iza' Zbyszynska in the years after the war

News of atrocities committed by the Nazi occupiers in Poland had reached the Allies before Sikorski's speech, but never on this scale.

Sikorski had even been advised by a fellow Polish politician to 'edit' the number of Jews murdered by the Nazis from 700,000 to 7,000, as the number was to shocking to be believed.

The information in Sikorski's speech would go down in history, but the man who got it there, a Swedish businessman named Sven Norrman, all but forgotten.

Sven Norrman, born in 1892, head of Swedish engineering company ASEA in Warsaw, Poland when the Germans invaded in 1939.

Norrman and a group of fellow Swedish expatriates working for corporations that would later become Swedish Match and mobile phone giant Ericsson, were evacuated but later returned.

Death sentence: Sigfrid 'Sigge' Häggberg is photographed in the Moabit prison in Berlin after Gestapo sentenced him and four other Warsaw Swedes to death in 1943, one year after their arrest

Brave: Nils Berglind was the executive director of LM Ericsson in Poland, the company that would later become mobile phone giant Ericsson

Hero: Carl Herslow, chief executive of Svenska Tändsticksaktiebolaget, later Swedish Match, in Poland

Released: Tore Widén, an engineer for Swedish Match, and the three other Warsaw Swedes, were later pardoned and returned to Sweden

Norrman and the Swedes lived as comfortable as it was possible in occupied Warsaw; electrics, phone lines and matches were, after all, goods and services the Germans had use of. Most of the time was spent living in a bunker in the Swedish Embassy.

Norrman, by now in his late 40s, spoke Polish and was well liked by his staff. Before the war, he had enjoyed hunts, collected Polish art, and had fallen in love with his secretary, a young Polish Jew named Gizela 'Iza' Zbyszynska.

Soon, Norrman and the others were witnessing increasing anti-Semitic violence and oppression of the Polish Jews. Norrman was able to photograph the very first implementation of the law requiring Jews to wear a yellow Star of David on their clothes, a practice which began in Poland.

He was later able to lie his way into the ghetto in Warsaw and took thousands of pictures of the horror within, before he was ushered out by a Jewish policeman for his own safety.

As the Nazis escalated their persecution and harassment, friends and colleagues of the Warsaw Swedes were jailed or simply disappeared, and when Norrman was approached by the Polish resistance army, he and the others decided to take a stand.

Travel in and out of occupied Poland was heavily restricted, but the men from neutral Sweden could move across the borders, so became the perfect couriers for Armia Krajowa - the Polish Home Army.

The Warsaw Swedes had been told by their employers in no uncertain terms than nothing was to be done that could anger the German occupiers.

The Swedish government wanted to avoid an invasion at all costs, and as a result, remaining neutral was paramount.

Moreover, the Warsaw Swedes knew that being discovered would lead to a certain death.

Despite this, Norrman and his fellow Warsaw Swedes began transporting goods with secret compartments in which they had hidden documents, photo-negatives and money.

Norrman, working under the alias 'Hjalmar', would become the most important connection between the Polish exile government in London and the resistance in Warsaw.

On May 16, 1942, the commander-in-chief of the Polish Armia Krajowa, General Rowecki, telegraphed to London: 'The Swedes take a very irregular route; still, they are fast and reliable and moreover can be entrusted with large deliveries. We cannot allow them to be exposed.'

Just days later, on May 21, Sven Norrman would take a very important suitcase with a hidden compartment from Warsaw to Stockholm.

It contained documents detailing the systematic slaughter of 700,000 Polish Jews by the Nazis, including 2,000 photonegatives of Nazi warcrimes in Poland, some taken by 51-year-old Norrman himself from inside the Warsaw ghetto.

Sven Norrman, right, with fellow Warsaw Swede Sigge Häggberg, centre, and Häggberg's son, left, on a bird hunt in Poland in the mid-1930s

This picture was taken by Sven Norrman in October 1939 in Wloclawek, where the law forcing Polish Jews to wear a yellow Star of David on their clothes was first implemented by the Nazis. The Jews were also banned from using the sidewalks. The picture bears the sarcastic caption: The master race's cultural endeavors in Wloclawek. Even woman and children walk in the street in Wloclawek with yellow marks on their backs'

A letter to King Gustav V of Sweden from Adolf Hitler regarding the four Swedes sentenced to death

Sikorski held his BBC speech a week later, and it did not take long before Gestapo began rounding up the Warsaw Swedes.

'Personally, I think the Germans listening in June 1942 grew worried and pissed off that the BBC was able to reveal in such detail exactly what was going on in Warsaw, and started to wonder how the hell the information had leaked,' says Staffan Thorsell, author of Warszawasvenskarna - De som lät världen veta (The Warsaw Swedes - The men who let the world know).

'Gestapo did not have to be Sherlock Holmes to figure out that the Swedes who travelled regularly between Warsaw and Stockholm could be involved.'

Seven Swedish men; Nils Berglind, Carl Herslow, Sigfrid 'Sigge' Häggberg, Tore Widén, Einar Gerge, Stig Lagerberg and Reinhold Grönberg were arrested by the Gestapo in July on Heinrich Himmler's order.

Sven Norrman was lucky. He was in Stockholm, and his mistress, Iza, had been able to get a message to him that Gestapo was looking for him. As he was the main suspect, it is likely that she saved his life.

Iza herself was also captured by Gestapo and taken to the Moabitprison in Berlin.

Four of the men, Berglind, Herslow, Häggberg and Widén were sentenced to death in July 1943. However, all seven would return safely to Sweden.

There are several theories why the four Warsaw Swedes were pardoned, and all are thought to have contributed to their final release in the autumn 1944.

Upon hearing that four Swedish men had been sentenced to death in Nazi Germany, the King of Sweden, Gustav V, wrote to Hitler himself and asked that the Warsaw Swedes be released.

Hitler admired royalty, and in particular those in countries that he perceived to be the birthplace of the 'Aryan' race, and personally assured King Gustav V that he would look into the matter in a letter sent at the end of July 1943.

The documents Sven Norrman transported from Poland to London via Sweden detailing the horror of the Nazi concentration camps and the use of gas chambers were translated into Swedish later in 1942, but banned by the then-government so as to not upset Nazi-Germany

Back in Warsaw: This photograph shows Sigge Häggberg, a Warsaw Swede who worked for Ericsson, in front of a bombed out telephone box in Warsaw in the summer of 1945

It is also known that Heinrich Himmler's personal physical therapist, Felix Kersten, a man who has been credited with saving the lives of some 60,000 Jews from the Nazi concentration camps, put in a good word with the Nazi top brass.

However, most importantly, Nazi Germany relied on neutral Sweden to supply them with iron and ball-bearings for their weapons, and it is highly likely that the four Warsaw Swedes were kept as leverage, to ensure that Sweden would fulfil their end of the weaponry bargain.

'To their contemporaries, the Warsaw Swedes were no heroes, but a risk factor,' Mr Thorsell adds.

'They had put Sweden in danger through their efforts for the Polish resistance army and by raising the alarm about the Holocaust.'

A book based on the documents Norrman transported to from Poland via Stockholm to London was published in Sweden during the war, but every single copy was confiscated by the government before it even reached the shelves.

The then prime minister, Social Democrat Per Albin Hansson, had personally ordered that the book, called 'A Polish Black Book On The German "New Order" In Poland', be collected from the publishers and banned.

After the war, Norrman had one priority. Find Iza.

Although Iza had lived as a Christian while in Warsaw, Gestapo uncovered her Jewish heritage shortly after her capture, and she spent the remainder of the war in a part of the Moabit prison reserved for Jews.

She survived her ordeal, and in 1945, Isa and Sven Norrman could be reunited in Warsaw.

In his book about the Warsaw Swedes, Mr Thorsell writes that Norrman most likely saved her life through bribes passed via Swedish Match's director in Germany to Himmler's man Walter Schellenberg.

Norrman divorced his wife, and the couple married in Sweden. Norrman was later rewarded the Armia Krajowa Cross by the Poles.

In an interview with Polish historian Jozef Lewandoski in the 1970s, Norrman explained why he decided to risk his life to help tell the world about the Holocaust.

'During my entire life I was a businessman. I liked my job and I was good in my field.