Rishi Sunak has said he will introduce emergency legislation to make sure his Rwanda plan is not blocked again and said “flights will be heading off in the spring as planned”.

After the Supreme Court ruling that the flagship asylum policy is unlawful, the prime minister said he has been working on a new international treaty with the East African nation to address the judges’ concerns and ensure it is “safe”.

He said: “This will provide a guarantee in law that those who are relocated from the UK to Rwanda will be protected against removal from Rwanda and it will make clear that we will bring back anyone if ordered to do so by a court.

Politics news – live: PM ‘prepared to change law’

“We will finalise this treaty in light of today’s judgment and ratify it without delay.”

Mr Sunak insisted the legislation would “end the merry-go-round” of legal challenges that have stopped flights from taking off since the controversial policy was announced in April last year.

“We need to end the merry-go-round. I said I was going to fundamentally change our country, and I meant it,” Mr Sunak said during a Downing Street press conference.

He said he would be taking the “extraordinary step of introducing emergency legislation” which will “enable parliament to confirm that with our new treaty, Rwanda is safe”.

But he also acknowledged that even if domestic laws are changed, the government could still face legal challenges from the European Court of Human Rights and vowed: “I will not allow a foreign court to block these flights.”

“If the Strasbourg court chooses to intervene against the express wishes of parliament, I am prepared to do what is necessary to get flights off,” he said.

In its ruling on Wednesday, the UK’s highest court said refugees sent to Rwanda would be at “real risk” of being returned to their country of origin, whether their grounds to claim asylum were justified or not – breaching international law.

It has fuelled calls from some Tory MPs to pull the UK out of the European Convention on Human Rights (ECHR) in order to push forward with the plan.

An eleventh-hour injunction from the ECHR stopped the first scheduled flights from taking off to Kigali last June, and no one has been deported since.

The Supreme Court judges said it is not only the ECHR which is relevant to their ruling, pointing out that the UK is signed up “other international treaties which also prohibit the return of asylum seekers to their countries of origin without a proper examination of their claims”.

However Mr Sunak was confident that his new plan will work.

He said he is “delivering” on his pledge to stop the boats, and the new treaty is “ready to go” to reassure the courts.

“We will clear the remaining barriers and flights will be heading off in the spring as planned,” he said.

The press conference came after Suella Braverman, who was sacked as home secretary on Monday, called for emergency legislation to “block off the ECHR and other routes of legal challenge”.

Conservative Party deputy chairman Lee Anderson said the government should “ignore the laws” and send migrants back the same day they arrive in the UK.