Asylum seekers who have experienced torture and trafficking were among the first people set to be deported to Rwanda, ITV News has been told.
As part of the government's controversial policy, 31 people were booked onto Tuesday's flight to the east African country.
On Saturday, 15 had their tickets cancelled. According to Care4Calais, a charity assisting this group, the 15 asylum seekers will now stay in the UK as there's evidence they have been tortured, trafficked or have a medical condition which makes deportation unsafe.
Care4Calais expressed concerned that had they not stepped in, most of these asylum seekers would have been flown to Rwanda.
A High Court ruling on Friday paved the way for a flight to go ahead on Tuesday - but an appeal against that decision is due to be heard on Monday.
The immigration policy has been heavily criticised by various groups, with even the Prince of Wales said to be “more than disappointed” by it, amid reports that he privately described the move to send migrants to Rwanda as “appalling”.
The Home Office plans to schedule more flights to follow Tuesday's departure. Up to 130 people have been notified they could be sent to Rwanda, and Care4Calais said they have met minors who say they have been served with such deportation notices.
The boss of the Public and Commercial Services union (PCS), which represents more than 80% of Border Force staff, said the “legality of these proposals” must be tested, but added there is also a need to debate “the morality and lack of humanity that the government is demonstrating” with its approach.
The first claim against the policy was brought by lawyers on behalf of some asylum seekers alongside the PCS, as well as groups Care4Calais and Detention Action, which are challenging the policy on behalf of everyone affected.
Mr Justice Swift on Friday ruled against granting a temporary block to the policy until a full hearing next month, but granted the claimants permission to appeal against his decision, suggesting Court of Appeal judges would hear the case on Monday.
PCS general secretary Mark Serwotka told Sky’s Sophy Ridge On Sunday programme: “We hope we win tomorrow in the Court of Appeal to stop the flight (on Tuesday).
“But, of course, the legality of these proposals will only be tested out at the full court hearing in July.
“We’re absolutely confident that in July, in line with what the UNHCR (UN Refugee Agency) said very graphically in court, we believe these proposals will be found to be unlawful.”
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He said home secretary Priti Patel would not ask civil servants to carry out the policy before its legality had been tested in court if she “had any respect, not just for the desperate people who come to this country, but for the workers she employs”.
Later on Sunday, scores of activists gathered near Brook House Immigration Removal Centre, Sussex to protest the deportations.
Many carried placards, some saying “it’s inhumane”, “we stand with you” and “stop the Rwanda flight”.Northern Ireland secretary Brandon Lewis defended the government’s policy, saying it aims to “break” the “business model” of people smugglers.
He added it would not be “appropriate” to comment on “rumoured” criticism from Charles.
A Clarence House spokesman has insisted Charles “remains politically neutral”, adding that “matters of policy are decisions for government”.