Hague insists Assad must face justice over new atrocity claims

London-based investigators examined more than 55,000 photographs said to show the emaciated corpses of victims, some of whom had eyes gouged out and showed signs of electrocution.

The former war crimes prosecutors’ report on the images – apparently smuggled out by a defector – indicated that “agents” of Bashar Assad’s regime are implicated. Damascus has denied allegations of abuses.

Speaking in the Commons, Mr Hague said Britain was doing “a great deal” to catalogue evidence of human rights violations.

“I’ve seen a lot of this evidence, it is compelling and horrific and it is important those who have perpetrated these crimes are one day held to account,” he said.

“The United Kingdom has done a great deal in the documentation of human rights abuses. Part of the support we have given to moderate political forces in Syria is to train human rights activists in the recording and documentation of crimes, many of which have therefore come to the world’s attention. We will do more of that.”

The report was commissioned by Qatar, which supports Syrian rebel and opposition groups.

One of the authors, Professor Sir Geoffrey Nice, told the BBC’s Newsday programme that the scale and consistency of the killings provided strong evidence of government involvement and could support a criminal prosecution.

Mr Hague also answered questions in the House on the United Nations’ decision to withdraw Iran’s invitation to the so-called Geneva II peace conference.

“It is very important to the future peace of Syria – whenever we come to that, whenever we are able to bring that about – to have Iranian commitment to it,” he said. “That is extremely important and that is why we have never opposed on principle Iranian involvement in the Geneva II process.

“I stressed last week in the Commons that it would be important for them to give some constructive signal of how they would approach Geneva, that they would approach Geneva II on the same basis as all other nations – to implement the Geneva communique of June 2012.

“It is a great shame they felt unable to do that publicly yesterday and that was why, to save the Geneva II process, the UN secretary-general rescinded the invitation he had issued on Sunday.”

In a telephone call with Iranian counterpart Mohammad Javad Zarif this afternoon, Mr Hague insisted Britain “remained open” to working with Tehran.

Russia, which backs the Assad regime, said the UN decision was a mistake but not a catastrophe.

Russia’s foreign minister Sergey Lavrov said it would have a negative impact on the United Nations’ image.

Mr Lavrov, speaking at a news conference, reaffirmed Russia’s argument that the presence of Iran was essential for the success of the conference which is set to begin in the Swiss resort of Montreux today.

Iran also sides with Assad and has provided advisers, money and materials since the uprising began in 2011. The invitation to Iran put the talks in jeopardy, with the Syrian opposition threatening to boycott the event entirely.

Russia has shielded the Assad regime from UN sanctions and continues to supply it with weapons throughout the civil war that has killed more than 130,000.

Mr Lavrov said that spurning Iran would deepen division lines in the Islamic world.

Iran branded the diplomatic about-face “deplorable” and demanded an explanation as to who had put pressure on Mr Ban to withdraw his invitation.