The British government was to send a first plane carrying failed asylum seekers to Rwanda on Tuesday despite last-gasp legal bids and protests against the controversial policy.
A chartered plane was to leave one of London's airports overnight and land in Kigali on Tuesday, campaigners said, after UK judges rejected an appeal against the deportations.
Claimants had argued that a decision on the policy should have waited until a full hearing on the legality of the policy next month.
Thirty-one migrants were due to be sent but one of the claimants, the NGO Care4Calais, tweeted that 23 of them had now had their tickets cancelled.
Those due to be deported include Albanians, Iraqis, Iranians and a Syrian, Care4Calais said.
Other claimants included the Public and Commercial Services Union, whose members will have to implement the removals, and immigration support group Detention Action.
PCS chief Mark Serwotka said on Sunday it would be "an appalling situation" if Tuesday's removals were subsequently found to be illegal at the full hearing.
Home Secretary Priti Patel should wait for the July hearing if she "had any respect, not just for the desperate people who come to this country, but for the workers she employs", Serwotka told Sky News.
Protesters gathered outside the Royal Courts of Justice and the Home Office on Monday.
In Geneva, UN refugee chief Filippo Grandi called the UK government policy "all wrong" and said it should not be "exporting its responsibility to another country".
Church of England leaders, including its most senior cleric the Archbishop of Canterbury, Justin Welby, reiterated their criticism of the policy as "one that should shame us as a nation".
"Our Christian heritage should inspire us to treat asylum seekers with compassion, fairness and justice, as we have for centuries," Welby and 24 other bishops wrote in Tuesday's Times newspaper.
"This immoral policy shames Britain."
"Evil trafficking" must be combatted by providing safe routes to the UK to "reduce dangerous journeys", The Times quoted the bishops as saying ahead of the letter's publication.
'Hate speech and discrimination'
Ms Patel and Prime Minister Boris Johnson insist the policy is needed to stop a flood of all-too-often deadly migrant crossings of the Channel from France.
"It's very important that the criminal gangs who are putting people's lives at risk in the Channel understand that their business model is going to be broken," Johnson told LBC radio on Monday.
"They're selling people falsely, luring them into something that is extremely risky and criminal."
Under the agreement with Kigali, anyone landing in the UK illegally is liable to be given a one-way ticket for processing and resettlement in Rwanda.
The government says that genuine asylum claimants should be content to stay in France.
And contradicting the UN refugee agency UNHCR, it insists that Rwanda is a safe destination with the capacity to absorb possibly tens of thousands of UK-bound claimants in future.
Doris Uwicyeza, chief technical adviser to Rwanda's justice ministry, pushed back against criticism of the human rights record of President Paul Kagame's government -- which is set this month to host a Commonwealth summit attended by Prince Charles and Johnson.
Rwanda's 1994 genocide made it particularly attentive to "protecting anybody from hate speech and discrimination", including gay people, she told LBC radio.
British newspapers reported that Prince Charles had dubbed the plan "appalling".
The reported comment prompted unnamed cabinet ministers to tell Queen Elizabeth II's heir to stay out of politics.
International NGO Human Rights Watch issued a public letter warning that "to this day, serious human rights abuses continue to occur in Rwanda, including repression of free speech, arbitrary detention, ill-treatment, and torture".
