Britain's intelligence chiefs refused to accept witness reports of the German massacre of Polish Jews in the Second World War and discounted the existence of the Holocaust, according to an authorised account based on official archives.

The intelligence chiefs thought that reports of the genocide of Jews in Poland, brought by two emissaries from Warsaw, lacked credibility. Their disbelief was one of the reasons why Winston Churchill was kept ignorant of the scale of the Holocaust at a time when decisive action might have been taken to try to stop the wholesale killings.

The dismissive response to the Holocaust reports in 1942 and 1943 is detailed in a remarkable publication of official intelligence records of the Second World War, sanctioned by the British and Polish governments.

Intelligence Co-operation Between Poland and Great Britain World War II highlights the successes of the Anglo-Polish wartime relationship, notably the extraordinary joint efforts by codebreakers to decipher Germany’s Enigma coding machine, used for all Berlin’s military communications throughout the war. The Poles were the first to break the Enigma codes.

However, the intelligence chiefs’ dismissal of the evidence of German genocide of Polish Jews provides an insight into one of the most controversial issues of the war. Anthony Eden, the Foreign Secretary, met one of the emissaries but was more interested in Polish-Soviet relations and future borders between the two countries than in any Allied action on behalf of murdered Jews. President Roosevelt also met the same emissary from Warsaw in Washington, but asked more questions about Polish resistance.

William Cavendish-Bentinck, chairman of the Joint Intelligence Committee, the main co-ordinator of intelligence in the 1939-45 war, summed up the views of the British intelligence hierarchy. He thought the Polish, and especially Jewish, reports on the German atrocities were not credible. According to the declassified intelligence archives, he stated in August 1943 that they were “exaggerating the German atrocities, and did so ‘to stiffen our resolve’”.

The two key emissaries from Warsaw, both witnesses of the slaughter, were Jan Karski, who came to London in November 1942, and Jan Nowak-Jezioranski, who arrived in December in 1943. Karski, a liaison officer of the Polish underground, told Cavendish-Bentinck about the mass murder of Jews in the Warsaw Ghetto and in a concentration camp called Belzec.

Nowak-Jezioranski said that some 3.3 million Polish Jews had been murdered from the beginning of the war until August 1943. His report in the intelligence archives says “the Germans used troops, tanks and artillery to liquidate the ghetto in Warsaw”. He handed over photos as evidence.

Although Karski, who was initially interrogated by MI5, wrote his own account of the disbelieving Allies after the war, the new history shows up the Whitehall brick wall that he and the other emissary faced when they tried to convince London and Washington of Germany’s Holocaust strategy.

The new official history says: “As chairman of the Joint Intelligence Committee and thanks to Enigma, Cavendish-Bentinck had access to the decrypted German police and SS reports which also mentioned the persecution and genocide of the Jews on the territories held by the Germans.

“This was a clear validation of Polish and Jewish information. Cavendish-Bentinck [a former Ambassador to Warsaw] was more interested in military intelligence on the Germany Navy than in the fate of dying Polish Jews.” It goes on: “As was the case with his political master, Anthony Eden, who was responsible for SIS [Secret Intelligence Service], he believed that only the swiftest possible end to the war could save the Jews of occupied Europe from complete annihilation.”

Roger Allen, a high-ranking Foreign Office official who worked closely with Cavendish-Bentinck during the war, “refused to believe that the Germans used gas chambers in Poland to murder people”.

At the end of August 1943, Allen wrote in a memo that he could “never understand what the advantage of a gas chamber over a simple machinegun or over starving people would be”. He said the recurring mentions of gas chambers in reports were “very general and tended to come from Jewish sources”.

The testimony of both Polish emissaries was kept secret. In the War Cabinet minutes concerning Karski’s account of the massacres, all references to the Jews were deleted, and when Eden wrote to Churchill on the subject, he also removed everything which mentioned Jews being murdered. Eden refused to let Karski report personally to Churchill because he felt it was “his duty to protect the elderly and overworked Prime Minister from too many petitioners”.

The official history, with a foreword by Tony Blair, is by the Anglo-Polish Historical Committee, set up five years ago to evaluate historical records. The first volume of findings is published this month.