Rishi Sunak is attempting to finalise a new pact with Rwanda after a central plank of the British government’s immigration plans was ripped up by the supreme court. A treaty, which would have to be ratified by the UK parliament, was being drawn up with the government in Kigali, the prime minister said. Whitehall sources said it could take more than a year for a treaty to be passed and then go through legal challenges in the courts. This would mean that flights may not take off for Rwanda under any new treaty before the next election. It comes after five judges at the supreme court unanimously rejected the government’s plan to send asylum seekers to the east African country. Deportation flights to Kigali will not take off despite the UK handing over more than £140m for a policy that has been stalled during more than a year of legal challenges. Addressing MPs, Sunak said he would “finalise” a new treaty with Rwanda in light of the supreme court’s judgment. In comments meant to reassure rightwing MPs calling for the government to make it harder for courts to intervene in immigration law, he said he was “prepared to revisit our domestic legal frameworks” if necessary. “The government has been working already on a new treaty with Rwanda and we will finalise that in light of today’s judgment,” he said. The supreme court’s judgment is expected to prevent the UK government from lawfully removing anyone to Rwanda, based on the evidence as it stands. Sunak’s official spokesperson said the UK government had been planning alternatives to the Rwanda deportation policy in the background before the supreme court judgment. Asked when work on a new treaty with Kigali had started, the spokesperson said: “The function of the court system means that they are, effectively, looking at an issue 15 months previously. The UK government has been very alive to that and we have been undertaking substantial work, including on this treaty, to ensure we are prepared to respond following the judgment.” Asked whether the former home secretary Suella Braverman had been incorrect to suggest there was no plan B to the Rwanda policy, Sunak’s spokesperson said: “That is probably a question for her. For our part, we have been working on the treaty and other measures. We have never said there is a single silver bullet to this approach and that is why we have followed a number of different avenues, and that is why crossings are down by a third year on year.” Speaking in the Commons, James Cleverly, the new home secretary, replacing the sacked Braverman, damped down hopes of rightwing Conservatives that the government could sidestep the European convention on human rights by introducing clauses to existing legislation. Yvette Cooper, the shadow home secretary, questioned whether the government would implement the “central clauses” of its Illegal Migration Act following the supreme court judgment. “What does this mean for the prime minister’s flagship legislation? Because he boasted about passing it only yesterday, but the government hasn’t actually commenced the central clauses for law, because without Rwanda – and frankly even with Rwanda – the policy doesn’t work and will just lead to an endless, ever increasing permanent backlog. “Can he confirm now he won’t, in fact, be implementing the central tenets of that law this year? And can he confirm that this means the prime minister’s pledges to introduce the new law to stop the boats and to also end hotel use will all be broken this year?” Cooper also suggested Cleverly had previously privately referred to the Rwanda plan as “batshit”. She told the Commons: “I don’t believe the new home secretary ever believed in the Rwanda plan. He distanced himself from it and his predecessor’s language on it. He may even on occasion have privately called it ‘batshit’.” Cleverly did not directly respond to the claim from the Labour frontbencher when he later answered her questions. A Rwandan government spokesperson, Yolande Makolo, said her government took issue with the ruling that Rwanda was not a safe third country for asylum seekers. However, Rwandan opposition politicians have criticised the proposals for wanting to send asylum seekers to a poor nation, saying a country such as the UK that is richer than Rwanda should not renege on its responsibility to host refugees fleeing persecution. Opposition leaders have been arguing Rwanda is a nation that still produces refugees itself and cannot be suitable for hosting those fleeing from persecution. Frank Habineza, the leader of Rwanda’s opposition Democratic Green party, has been vocal in speaking out against the policy. He told the Guardian that Rwanda could not afford to accommodate asylum seekers turned away from Britain. “The UK should take responsibility and host these asylum seekers because this is a nation they chose. If the UK wants to send away these people, let it send it to another European nation at least,” Habineza said.