The archbishop of Canterbury has clashed with ministers after branding the government’s flagship illegal migration bill as “morally unacceptable” legislation that will “damage the UK’s interests and reputation at home and abroad”. In a withering attack upon Rishi Sunak and Suella Braverman’s plan, Justin Welby said it would not fulfil the prime minister’s pledge to “stop the boats”, ignored the key causes of the movement of refugees, and could break the system of international cooperation that promised to help those fleeing war, famine and conflict. “[The bill] is isolationist, it is morally unacceptable and politically impractical to let the poorest countries deal with it alone and cut our international aid,” he said. “This is an attempt at a short-term fix. It risks great damage to the UK’s interests and reputation at home and abroad, let alone the interests of those in need of protection or the nations who together face this challenge.” Jolted into a response, Robert Jenrick, the immigration minister, said the most senior cleric in the Church of England was “wrong” and defended the bill’s aim of criminalising, detaining and removing people who arrive in the UK in small boats to their home country or a third country such as Rwanda. In his first intervention during a House of Lords debate on the bill, Welby said it did not address the two key causes of international migration: the climate crisis and war. The archbishop cited forecasts from the Intergovernmental Panel on Climate Change estimating that the climate crisis alone would lead to at least 800 million more refugees a year by 2050. “Even if this bill succeeds in temporarily stopping the boats – and I don’t think it will – it won’t stop conflict or climate change,” he said. The bill could lead to the breakdown of the international pledge to aid refugees, he said. “The UNHCR has warned that it could lead to the collapse of the international system to protect refugees. Is that what we want the United Kingdom’s contribution to be in our leadership?” he said. “It ignores the reality that migration must be engaged with at source as well as in the Channel,” he said. He acknowledged that reforms to immigration laws were needed to “destroy the evil tribe of traffickers” facilitating the small boat crossings. But, he said: “The tragedy is that without much change, this is not that bill.” The bill instead treated people-smugglers as “rationally trained economic actors and not appalling criminals”, he said, adding that the government should not delay setting up “safe and legal routes” for people seeking asylum in the UK. “There must be safe, legal routes put in place as soon as illegal or unsafe routes begin to be attacked. We cannot wait for the years that will take place before that happens.” Welby said he planned to table amendments to the bill at committee stage, which would include plans that he said were missing in the current bill to combat people-traffickers and to update the 1951 UN refugee convention. Dozens of peers criticised the bill including Labour’s spokesperson Vernon Coaker, who said: “We have a government playing fast and loose with our place in the world and our respect for international law. This must change.” The former head of the British army Sir Richard Dannatt said the bill was “vicious” and to illustrate its failings used the case of an Afghan pilot who had arrived in the UK on a small boat. “The viciousness, and I use that word quite advisedly, of this bill offends many people’s moral position. It runs the risk of offending Britain’s standing in the world,” he told BBC Radio 4’s World at One. In response to Welby, Jenrick told the BBC: “There’s nothing moral about allowing the pernicious trade of people smugglers to continue … I disagree with him respectfully. “By bringing forward this proposal we make it clear that if you come across illegally on a small boat you will not find a route to life in the UK. That will have a serious deterrent effect.” Downing Street insisted the bill was “compassionate and fair”. Asked to respond to Welby’s description of it as “morally unacceptable,” the prime minister’s spokesperson said: “The prime minister does not think it is compassionate or fair to allow people who are jumping the queue over some of the most vulnerable people who are seeking to come here through safe and legal routes. “We think it is the compassionate and fair thing to do.” The passage of the bill is expected to continue into the summer, when critical peers will seek to amend it at the report stage. The bill received the backing of Lord Howard of Lympne, the former Conservative party leader and a former home secretary, who said measures in the legislation were vitally needed because of France’s refusal to agree a returns deal. He said the “first duty of a government is to protect the borders of the state” and the bill “represents the best available means of achieving that objective”. Howard acknowledged there were “legitimate arguments” in favour of introducing more safe and legal routes for refugees, but he said they were “irrelevant” to the need to introduce fresh measures to prevent crossings.