David Cameron has been widely condemned for failing to respond to the execution of 47 prisoners in Saudi Arabia on Saturday.
Criticism has increased of the UK's close relationship with Saudi Arabia in recent months in response to flagrant human rights abuses. The country's legal system is based on Sharia law, and the death penalty can be imposed for a wide range of offences including rape, murder, adultery and blasphemy.
On January 2, 47 executions were carried out, including that of prominent Shiite cleric Sheikh Nimr al-Nimr; a strong critic of the Saudi royal family. On Sunday, Iran severed diplomatic ties with Saudi Arabia over al-Nimr's death.
Cameron has not yet commented on the executions, despite calls for him to do so, and his silence has now been branded "utterly shameful" by leading human rights campaigner Peter Tatchell.
The UK government's response was "completely immoral," Tatchell said.
Speaking to the Independent, former Lib Dem leader Lord Ashdown warned that ongoing conflict between Sunni and Shia Muslims "poses a far greater danger in the long term" than ISIS.
"These executions are deeply, deeply destabilising to the very delicate situation that exists in the Middle East," he said.
"The West, including the UK government, is only just realising the danger of this and its implications for long term peace in the region."
Shadow human rights minister, Andy Slaughter, condemned the UK's relationship with Saudi Arabia. In a letter to justice secretary Michael Gove, he said, "It is not right that the UK should be actively cooperating with a justice system that shows such flagrant disregard for the most basic human rights and the rule of law."
Human rights group Reprieve also urged Britain not to "turn a blind eye to such atrocities".
"The Saudi government is continuing to target those who have called for domestic reform in the kingdom, executing at least four of them today," said death penalty team director Maya Foa.
"There are now real concerns that those protesters sentenced to death as children could be next in line to face the swordsman's blade. Saudi Arabia's allies – including the US and UK – must not turn a blind eye to such atrocities and must urgently appeal to the kingdom to change course."
UK Minister for the Middle East Tobias Ellwood said today in a statement: "The UK is firmly opposed to the death penalty. We have stressed this to the Saudi authorities and also expressed our disappointment at the mass executions."