Liz Truss has been accused by the UN’s refugee chief of making “untrue” statements after claiming that critics of the UK government’s Rwandan removals policy have failed to come up with alternative policies.

Filippo Grandi, the United Nations high commissioner for refugees, said the foreign secretary was wrong because the UN had offered “many, many suggestions” instead of sending people to the east African state, which he said “violates the fundamental principles of refugees”.

The clash comes as the government prepares to send the first flight of asylum seekers to Rwanda on Tuesday. Government sources said the plane was prepared to take off with a single refugee onboard if necessary because the costs estimated at £500,000 cannot be reimbursed.

Government insiders are concerned there could be fewer than five people on the flight following a flurry of last-minute legal challenges. Since last week, the number of refugees due to be on the flight has fallen from 130 to seven on Tuesday morning.

Truss told Radio 4’s Today programme that critics such as leading figures in the Church of England should come up with alternative policies. “Those people need to suggest an alternative policy that will work. Our policy is completely legal, it’s completely moral,” she said.

In response, Grandi told the Guardian: “This is simply untrue because we have offered many, many suggestions to the British government on how to simplify and accelerate procedures and maintain their fairness.”

A Rwandan government spokesperson, Yolande Makolo, told a press conference on Tuesday that there were “misconceptions” about what Rwanda is like and “some of this is perpetuated by the media”.

At a briefing at the ministry of foreign affairs in Kigali, she said: “Tomorrow when the first first flights land here in Kigali the new arrivals will be welcomed and looked after and supported to make new lives here. We will provide support with their asylum applications, including legal support, translation services and we will provide decent accommodation.”

She added: “We are ready to receive thousands over the life of this partnership.”

When asked about the “outcry” about the scheme, Makolo said Rwanda was entering into the partnership for the “right reasons”.

“We have the experience. We want Rwanda to feel a welcoming place,” she said. “We understand the opposition to this but we are asking for this programme to be given a chance to be the solution. People are suffering, the asylum system is broken and taken advantage of by criminal gangs, they are exploiting people.”

Pressed by the Guardian on whether migrants would face restrictions on their movement or curfews at the accommodation facilities, she said: “There will be in-house rules as there are in many accommodation centres in many parts of the world, there will be a few guidelines on how the space will be used.”

She said there would be “basic house-keeping rules for a shared facility that is home to many people” but did not elaborate on what these might be.

Speaking at a press conference in Geneva, Grandi said the UN had advised that the UK could finalise agreements with countries of transit through which refugees travel and strengthen burden-sharing between European countries. He advised the UK government to work with the EU to solve the problem.

“Cooperation is possible without resorting to this type of arrangement such as the one with Rwanda that – and I won’t repeat it anymore but we said it many times – violates the fundamental principles of refugees,” he said.

In her interview, Truss admitted there might be few people on the plane to Kigali. Asked if there could be no one on this flight, she said: “There will be people on the flight, and if they are not on this flight they will be on the next flight … I don’t have a figure. The important point is the principle.”

But a government source said the flight would take off even with just one refugee onboard because the government would not “get anything back in cost terms by cancelling the flight”, which reports have claimed could cost as much as £500,000.

Regarding the costs, a government source said: “The broken asylum system currently costs £1.5bn a year. We are spending £5m a day on hotel costs. Can people really put a price on the cost of saving human lives and securing our nations borders?”

Downing Street released a statement from Boris Johnson arguing that critics of the policy are aiding criminal gangs who thrive by operating small boats for asylum seekers across the Channel. “I think that what the criminal gangs are doing, and what those who effectively are abetting the work of the criminal gangs are doing, is undermining people’s confidence in the safe and legal system, undermining people’s general acceptance of immigration,” Johnson said before a meeting of his cabinet.

The supreme court on Tuesday rejected an appeal bid over a judge’s refusal to block the removal of an asylum seeker due to be deported to Rwanda. Two more cases are being considered by the court before the flight takes off.

In a letter to the Times, the archbishops of Canterbury and York, as well as the other 24 bishops that sit as lords spiritual in the House of Lords, said the policy “should shame us as a nation”.

The policy has also drawn widespread condemnation from beyond the Church of England – where the archbishop of Canterbury, Justin Welby, said it “could not stand the judgment of God”. Prince Charles was reported to have privately described the plan as “appalling”.

Care4Calais, one of the charities that brought the defeated legal appeal to halt the flight, said just seven migrants expecting to be removed still had live tickets.
Clare Moseley, the head of the charity, said the government could not explain why the policy would work. “Why is our government so very determined to spend … so much taxpayer money on a brutal plan that shames us all and send refugees to Rwanda?”