On the eve of the first scheduled departures, the British courts are examining last-minute appeals against the government's plan to send migrants who arrived illegally in the United Kingdom to Rwanda.
A first flight carrying about 30 of them is scheduled for Tuesday, four days after an appeal by refugee associations failed.
However, the plaintiffs, including the associations Care4Calais and Detention Action, have filed an appeal, which will be heard on Monday.
Among the plaintiffs is the civil service union PCS, whose members include customs officers who are supposed to carry out the deportations.
Pointing out that the High Court plans to examine the legality of the government's plan in detail in July, PCS general secretary Mark Serwotka said on SkyNews on Sunday: "Imagine being told to do something on Tuesday, which in July is deemed illegal. That would be a terrible situation.
On Monday, the High Court is also due to hear another appeal, brought by the refugee aid association Asylum Aid.
By sending asylum seekers more than 6,000 kilometers from London, similar to Australia's policy, the government hopes to deter the growing number of illegal arrivals in the country.
Since the beginning of the year, more than 10,000 migrants have illegally crossed the English Channel to reach the British coast in small boats, a considerable increase compared to previous years, which were already record numbers.
Saying he wants to go after the smugglers, Prime Minister Boris Johnson says the strategy will help "break the business model of these ruthless criminals."
The UN strongly condemned the strategy at a hearing Friday at the High Court in London.
Office of the High Commissioner for Refugees (UNHCR) lawyer Laura Dubinsky said the UN agency was concerned about the risk of "serious and irreparable harm" to migrants.
"UNHCR is not involved in the arrangement between the UK and Rwanda, despite the minister of state's claims to the contrary," she also stressed, accusing the government of lies.
James Nichol, a lawyer and administrator of the Care4Calais association, denounced a "brutal" policy aimed at "people who come from war-torn countries" and "are already traumatized".
Adding to the controversy, comments attributed to Prince Charles have caused a stir. The heir to the throne privately described the government's plan as "appalling", The Times reported on Saturday.
A source told the newspaper that he has heard the 73-year-old prince privately express his opposition to the policy several times and fears it will overshadow a Commonwealth meeting to be held in Rwanda starting June 20. On that occasion, he will represent his mother, Queen Elizabeth II.
Clarence House, which manages Prince Charles' communications, declined to comment on the remarks but stressed that the heir to the throne "remains politically neutral," as required by his role in the royal family.
In Kigali, Prince Charles and Boris Johnson are due to meet with President Paul Kagame, who has ruled Rwanda since the end of the 1994 genocide in which 800,000 people died, according to the UN. His government is regularly accused by NGOs of repressing free speech, criticism, and political opposition.
On Friday, 23 NGOs called on Commonwealth leaders to pressure Rwanda to release critics of the government and allow greater freedom of expression.