Government promises about the treatment of asylum seekers who could be deported to Rwanda are merely “aspirational”, the country’s highest court has heard.

The United Nations refugee agency told the Supreme Court that the pledges were “no sufficient answer” to “basic and fundamental defects” in the Rwandan system.

The Home Office is challenging a June Court of Appeal ruling that the UK’s £120million plan to send asylum seekers to the African state to have their claims processed is unlawful.

The policy is seen as key to Rishi Sunak's promise to "stop the boats" by creating a deterrent to crossing the Channel.

The UK Government previously argued a memorandum of understanding agreed between the two countries provided assurances that all those sent there would have a “safe and effective” refugee status determination procedure.

Assurances were also made that people deported to Rwanda would be given “adequate accommodation”, food, free medical assistance, education, language and professional development training and “integration programmes”.

However, Angus McCullough KC, for the UNHCR, said in written submissions: “The assurances and commitments given by the government of Rwanda do not suffice to establish an accessible, reliable or fair asylum system in Rwanda.

“UNHCR in any event considers the assurances given in this case insufficient.

“The assurances are aspirational in nature, particularly in relation to such matters as ‘appropriately trained’ decision-makers and ‘objective and impartial’ decisions.”

Five Supreme Court justices were told that asylum seekers transferred to Rwanda under a previous arrangement with Israel were “routinely and clandestinely expelled”, prevented from making asylum claims and faced “grossly intimidating treatment”.

The country was described as an “authoritarian, one-party state” by a barrister for the asylum seekers in the claim.

Mr McCullough also said in written submissions: “The Home Office’s response to the evidence of inadequacies and failures in the system is to assert that, in light of the assurances, evidence of past problems is ‘at best, peripherally relevant’… That approach has no basis in principle or in the evidence.”

At the start of the hearing on Monday, Sir James Eadie KC, for the Home Office, said the policy to remove people to “a country less attractive” than the UK, “but nevertheless safe”, was lawful.

Sir James said that asylum seekers’ rights of review and appeal were “embedded” in the deal with Rwanda, which also “guaranteed” access to legal support.

In written arguments, he said transfers to Rwanda would “take place only with the consent of the Rwandan authorities and numbers will, in the first instance, be low”.

The hearing before Lords Reed, Hodge, Lloyd-Jones, Briggs and Sales is due to end on Wednesday, with a judgment expected at a later date.