Several flights a month will deport asylum seekers to Rwanda “indefinitely”, the home secretary has said, as he argued that the £1.8m a person cost of the scheme was justified. James Cleverly, in his first interview since the government’s plan was approved by parliament on Monday, said he had booked a succession of initial flights and was preparing to order the detention of people seeking refuge in the UK so they could be sent to east Africa. Aiming to defuse criticism that the Rwanda plan was a waste of money, which the National Audit Office said could exceed £580m by the end of the decade, Cleverly said it would “massively reduce” the costs of housing asylum seekers in the UK. Analysis from the Refugee Council suggests the Rwanda policy could cause “a system meltdown”. The home secretary’s most detailed comments on the scheme were made on a visit to Lampedusa, the Mediterranean island that has been the first European landing point for hundreds of thousands of people escaping war and famine in Africa. Cleverly told the Guardian: “The prime minister has made clear we’ve got facilities, we’ve got plans in place, that will facilitate multiple flights per month, indefinitely. “We have booked a number of initial flights. And we have got the facilities to continue booking flights on a regular drumbeat, through the summer.” Royal assent, when the monarch formally agrees to make a bill an act of parliament, is expected to be granted on Thursday, according to Whitehall sources. This is expected to be swiftly followed by the ratification of the Rwanda treaty, which is the government’s attempt to respond to criticism from the supreme court. The troubled scheme, which is two years old, has so far sent no one to Rwanda. But its success is an important component of Rishi Sunak’s plan to revive his fortunes before a general election. If the Rwanda scheme was to fulfil its aim of deterring people from travelling to the UK in small boats, there must be regular and repeated deportation flights, Cleverly said. “Obviously, people will take a huge amount of interest in the first flight,” he said. “But we recognise that it is when people say, ‘Hang on a minute, there’s another flight and then another one and then another one’ – that regularity is what will trigger the deterrent effect in the Channel.” Cleverly said the cost of the Rwanda treaty, which would pay the Paul Kagame government £490m once 300 people have been sent, would be justified if small boats stop coming to the UK. “A lot of this expenditure is linked to the success of the project,” he said. “And the project’s success will itself massively reduce the cost to the British government, for example, of the asylum accommodation bill. So when we’re looking at the costs, we’ve got to look at it in the context of the alternative reality.” Lawyers representing asylum seekers who have previously faced possible removal to Rwanda are considering legal action once the legislation is passed, the Guardian understands. They are expected to mount a challenge on the grounds that the law is incompatible with the European convention on human rights. Cleverly declined an invitation to say that his plans were legally watertight. “We have, of course, given a huge amount of thought to the things that have happened in the past and what might happen in the future,” he said. “We absolutely know that our planning is robust.” Last week, the Italian prime minister, Giorgia Meloni, signed three agreements with Tunisia to curb travel to Italian islands such as Lampedusa. In a harbour on the island, one sea rescue worker criticised Cleverly for travelling to the tiny Sicilian island to promote his Rwanda deal. Austin Cooper, an island mediator and care coordinator on Sea Watch’s rapid response vessel that helps to rescue those in trouble at sea, said: “Instead of criminalising people on the move and taking away safe and legal routes to arrive, Cleverly should be making it possible for the people who need to reach safety and be welcomed with dignity, not getting tips from European leaders on how to make the Channel even more dangerous.” The Refugee Council called for immigration laws to be repealed to avoid catastrophe, as it estimated recent changes to the system may end up emptying the public purse of up to £17.1m a day – or about £6.2bn a year – in accommodation costs alone. The charity also suggested 115,575 asylum seekers could be stranded in a “permanent limbo” by the end of 2024. Under the Illegal Migration Act, which became law last year, asylum seekers who arrive by small boats are banned from applying as their cases are deemed “inadmissible” by the government. The law also puts a duty on the home secretary to arrange for their removal, though this element of the legislation is yet to be brought into force. Enver Solomon, the chief executive of the Refugee Council said: “This report lays bare the immense cost, chaos and human misery that the Illegal Migration Act and Rwanda plan will unleash. “The government has recklessly brought in this misguided legislation without any apparent thought to its staggering costs and long-term consequences. What is happening is of a different nature to anything seen before because it shuts down the entire asylum system, which is unprecedented.” More than 400 migrants arrived in the UK on the day five people, including a child, died while trying to cross the Channel, it emerged on Wednesday. The crossings took place as the tragedy off the coast of northern France unfolded, only hours after parliament passed the Rwanda legislation. A Home Office spokesperson said: “Flights to Rwanda will take off in 10 to 12 weeks, creating the deterrent needed to end the criminal exploitation of migrants by the people smuggling gangs.”