British Foreign Secretary Liz Truss insisted the first flight to Kigali would take off no matter how many people were on board, after 23 of the 31 migrants had their tickets cancelled.
"We're expecting to send the flight later today," she told Sky News, as fresh protests were held at a detention center near London Gatwick airport.
Truss said she was unable to confirm how many people would be on the charter flight to Kigali, which was due to leave from an undisclosed airport on Tuesday night.
But she said the policy, which the UN refugee agency has also criticized as "all wrong", was vital to smash the business model of human-trafficking gangs exploiting vulnerable migrants.
Campaigners supporting migrants and a union representing Border Force workers who will have to carry out the policy failed in a legal challenge to stop the deportations.
After the latest attempt was thrown out on Monday, the two senior-most clerics in the Church of England and 23 bishops called the policy "immoral" and said it "shames Britain".
"They (migrants) are the vulnerable that the Old Testament calls us to value," Archbishop of Canterbury Justin Welby and Archbishop of York Stephen Cottrell wrote in a letter to The Times.
"We cannot offer asylum to everyone, but we must not outsource our ethical responsibilities, or discard international law -- which protects the right to claim asylum."
Truss, though, hit back. "The people who are immoral in this case are the people traffickers trading on human misery," she said.
"Our policy is completely legal. It's completely moral," she added, accusing critics of having no alternative plan.