The government's failure to ban Iran's Islamic Revolutionary Guard Corps more than four months after MPs voted for it was challenged on Tuesday.
The House of Commons unanimously passed a motion in January to urge the government to proscribe the IRGC as a terrorist group.
Although the vote was not binding, it put pressure on ministers to respond to violence against protesters in Iran by security forces controlled by the IRGC.
Peers in Westminster have now called on the government again to take action against the military body, which has been proscribed by allies such as the US.
“It’s now been more than four months since the House of Commons passed a resolution universally in favour of proscription of the IRGC," Labour's Lord Walney said.
“This is an organisation which is committed to armed resistance against Israel, exactly the same grounds on which the government has proscribed Hezbollah.
“Our intelligence services are clear that it is committed to kidnaps and killings on UK soil.
“There must be an argument within government not to do this, so can the minister enlighten the House on what this is, because it seems very obvious that it should be proscribed to many members across both Houses?”
Labour frontbencher Lord Collins of Highbury said: "This is not good enough.
“We’ve had ministers and prime ministers support the objective of banning this organisation, which is a threat not only to the citizens of Iran, but also to the citizens in this country.
“It’s about time we acted, rather than just talked.”
Former Tory Cabinet minister Lord Pickles said there was a feeling of “groundhog day” about the Government’s responses to questions about banning the IRGC.
“We seem to be going around the Houses time and time again," he said.
“This is an organisation that pays for the people that send missiles into Israel into residential areas, use children as human shields, they pay for organisations that have murdered a British mother and her children.
“What more does the IRGC need to do for the government to proscribe it?”
UK considers proscribing Iran’s Revolutionary Guards - video
Labour peer Lord Harris of Haringey said there was “clearly a row going on in government”, claiming that the Foreign Office is vetoing a ban on the IRGC against the will of all other departments, and accused the government of “shilly-shallying” on the issue.
Home Office minister Lord Sharpe of Epsom said: “The government takes the threat of the IRGC very seriously, and continues to condemn its actions.
"And the government will always consider the full range of powers available, including our robust anti-terrorism powers such as the proscription tool, where appropriate, to address the threat posed by Iran and the IRGC.
“Over 300 Iranian individuals and entities have been sanctioned for activities which include human rights violations, including 70 since October last year.
“Other activities would include nuclear proliferation, support to Russia, various other regional activities.
“As an entity, the IRGC was designated in its entirety under the Sanctions and Anti-Money Laundering Act of 2018.
"More than 30 new designations of IRGC-related organisations and officials have been made by the FCDO since October 2022.
“The government does keep the list of proscribed organisations under review.”
Lord Sharpe said the government would not comment further on proscription to avoid creating expectations and reduce the risk of an organisation taking evasive action before any potential proscription order.
Meanwhile, the Bishop of Chelmsford, the Rt Rev Dr Guli Francis-Dehqani, called for safe routes to the UK for Iranians facing religious persecution.
“In a recently published joint annual report by Article 18 and other Christian organisations, the IRGC’s increasing involvement in the crackdown against peaceful Christian activities was highlighted for the second year in a row," she told Lords.
“Other religious minorities and peaceful protesters also report violent treatment during arrest and detention, as well as interference of the IRGC’s intelligence branch in court proceedings to ensure harsher sentences against those who are accused.”
Bishop Francis-Dehqani said there was more the government could and should do beyond proscription.
She asked whether it would consider offering a “safe route scheme for those from Iran who have suffered persecution in the form of arrest and imprisonment on account of their faith".
“I can’t comment on that but I will make sure her views are taken back," Lord Sharpe said.
