The first flight removing asylum seekers to Rwanda will take off no matter how few people are on board, the Foreign Secretary has indicated as she defended the policy as “completely moral”.

Liz Truss insisted “some” individuals would be on the the plane to east Africa on Tuesday evening but could not say how many as she insisted the scheme is both legal and “value for money”.

The archbishops of Canterbury and York described the “immoral” plan as one that “shames Britain” as they added to vehement criticism from opposition parties.

But Ms Truss struck back, telling them to come up with their own policy, amid suggestions the first flight could contain only seven people or fewer.

“We are expecting to send the flight later today,” she told Sky News.

“I can’t say exactly how many people will be on the flight but the really important thing is we establish the principle and we start to break the business model of these appalling people traffickers who are trading in misery.”

On Monday three Court of Appeal judges upheld a High Court ruling that the removal could go ahead, rejecting an appeal by two refugee charities and the Public and Commercial Services union (PCS).

Ministers had initially planned for up to 130 people to be on the initial flight but Care4Calais, one of the charities that brought the legal appeal, said that just seven migrants expecting to be removed still had live tickets.

And three further challenges brought by individuals who face removal are expected to be heard at the High Court on Tuesday.

Ms Truss insisted: “There will be people on the flight and if they are not on this flight they will be on the next flight.”

She did not deny estimates that a charter flight could cost hundreds of thousands of pounds, instead saying she “can’t put a figure” on the expense.

But Ms Truss stressed that ministers must “reduce the cost over time of illegal immigration” as they struggle to tackle small boat crossings of the English Channel.

“It is value for money,” she insisted to Sky.

Archbishop of Canterbury Justin Welby and Archbishop of York Stephen Cottrell said the policy “should shame us as a nation”.

“This immoral policy shames Britain,” they said in a letter to the Times, which was also signed by the bishops of London, Durham, Exeter, Birmingham and Manchester.

But Ms Truss said she disagreed with their criticism, insisting the “people who are immoral in this case are the people traffickers trading in human misery”.

And in a message to her critics, she told Sky: “Those people need to suggest an alternative policy that will work.

“Our policy is completely legal, it’s completely moral.”

Ms Truss declined to predict the numbers of people who will be forcibly removed to Rwanda this year during the scheme, which is part of a £120 million five-year deal with Rwanda.

Instead she told BBC Radio 4’s Today programme: “They will be significant.”

Detention Action, the other charity involved in the appeal, said it was “disappointed” with the Tuesday’s legal ruling but would continue to press ahead with a full legal challenge to the policy due to be heard next month.

“Throughout these injunction hearings, we have heard powerful evidence against this policy and we are confident that, when our full legal challenge is heard in the High Court in July, this Government will be forced to abandon this policy and bring back anyone who is ultimately removed,” deputy director James Wilson said.