Pressure is mounting on David Cameron over his refusal to declare ISIS atrocities committed against Christians and other religious minorities a genocide.

A group of peers will support an amendment to the immigration bill tabled by Lord Alton, who is a Christian, which would see the conflict labelled as such. A vote is expected later on Monday.

In a letter to Lord Alton last week, David Cameron said there was "no need" to reconsider the government's current position of not using the term.

"Not only are the courts best placed to judge criminal matters but their impartiality also ensures the protection of the UK Government from the politicisation and controversies that often attach themselves to the question of genocide," he wrote.

"The Foreign and Commonwealth Office have recently reviewed this long-standing position and I agree with their conclusion that there is no need to reconsider it at this time."

In response Alton, who is a seasoned campaigner against Christian persecution, said political equivocation "while a genocide is underway is morally indefensible".

"No more excuses - the Government must act," he said.

The call comes after the US administration unexpectedly declared ISIS had committed genocide. It followed a unanimous vote in the House of Representatives on Monday and comes after the European Parliament passed a similar motion in February.

In addition to Alton, other peers who support the amendment include Caroline Cox, Helena Kennedy, Emma Nicholson and Michael Forsyth. In a letter pleading for members' support Cox said: "It is noteworthy that, in the past two years, two serving foreign secretaries have lamented the failure of the international community to decry the genocides in Bosnia and Rwanda quickly enough, despite overwhelming and compelling evidence. We have an opportunity to prevent history from repeating itself."

David Cameron has received a number of calls prior to this to recognise the atrocities as genocide. In December 75 peers wrote to Cameron to plea for his support, including Charles Guthrie, the former head of the UK military, and Jonathan Evans, the former head of MI5.

The Archbishop of Canterbury, Justin Welby, has not yet explicitly declared the conflict a genocide, nor has the Catholic Archbishop of Westminster, Vincent Nichols.