Britain was accused of "genocide denial" today after the disclosure of Foreign Office documents revealing the government's refusal to recognise the so-called Armenian massacre of 1915, in which up to a million people died.

The documents, dating back over the last 15 years, say Anglo-Turkish relations are too important to be jeopardised by the issue because "Turkey is neuralgic and defensive about the charge of genocide".

One Foreign Office briefing for ministers conceded that the British government "is open to criticism in terms of the ethical dimension", but goes on to say: "The current line is the only feasible option" owing to "the importance of our relations (political, strategic and commercial) with Turkey". The 1999 briefing said: "Recognising the genocide would provide no practical benefit to the UK."

Britain's stance, stretching back over Labour and Tory administrations, was called a cynical "genocide denial" by Geoffrey Robertson, the QC who served as first president of the UN war crimes court for Sierra Leone. Robertson was commissioned by Armenian expatriate groups in London to review the foreign office files, obtained in heavily redacted form from freedom of information requests. He published a report today which says: "Parliament has been routinely misinformed by ministers who have recited FCO briefs without questioning their accuracy."

The allegation that the Armenian massacres during the first world war were a form of genocide, carried out by the Ottoman empire, is a bitterly contested issue that has soured relations between Turkey and Armenia. The border between the two countries was re-opened last month after being closed since 1993, thanks to an accord which includes a promise to set up a commission of historians to re-examine the affair. Turkish and Armenian parliaments still have to ratify the accord.

The Foreign Office documents include advice in 1995 to the then Tory foreign minister, Douglas Hogg, that he should refuse to attend a memorial service for the victims, and attempts to encourage the idea that historians were in disagreement over the facts. The government refused to include the Armenian massacres as part of holocaust memorial day.

Robertson's report says: "There is no doubt that in 1915 the Ottoman government ordered the deportation of up to 2 million Armenians … hundreds of thousands died en route from starvation, disease, and armed attack."

The1948 genocide convention was drawn up with the specific case of the Armenians in mind, he says, and most scholars and European parliaments have described their fate as genocide. "But recent British governments … have resolutely refused to do so," resorting instead, he says, to the legally meaningless expression that "insufficiently unequivocal evidence" of genocide exists.

Britain is a keen supporter of Turkey's attempts to join the EU. But the Armenian question has become a touchstone for critics, who argue that Turkey should not be allowed into the EU until it admits the truth about its past. Turkey refuses to allow any of its citizens to call the Armenian massacres genocide. When Nobel prize-winning writer Orhan Pamuk did so, he was charged with "insulting Turkishness" in 2005, although the justice ministry refused to let a trial proceed, following an embarrassing international outcry.

Three scholars, Ahmet Insel, Baskin Oran and Cengiz Aktar, and a journalist, Ali Bayramoglu, published an open letter, inviting Turks to sign an online petition supporting its sentiments. It reads: "My conscience does not accept the insensitivity showed to and the denial of the Great Catastrophe that the Ottoman Armenians were subjected to in 1915. I reject this injustice and for my share, I empathise with the feelings and pain of my Armenian brothers. I apologise to them."

But while academics edge towards openness, Robertson says Britain's official policy has merely been "to evade truthful answers, because the truth would discomfort the Turkish government".