Asylum seekers should not be deported to Rwanda before the policy has been fully tested in the courts, a union leader has said, predicting the first removal flight will not take place as planned this week.

An initial deportation flight under the plan to remove people who arrive in the UK via unofficial routes to the east African country is scheduled for Tuesday after a high court judge ruled on Friday that it could go ahead.

Mark Serwotka, the general secretary of the PCS union, which represents more than 80% of Border Force staff, said he was hopeful an appeal in the high court on Monday would stop the flight.

He also argued that the home secretary, Priti Patel, should not ask officials to implement the policy before its legal position was fully tested.

He told Sky’s Sophy Ridge on Sunday programme: “Imagine if you’re a civil servant now in the Home Office, who goes to work because you want to see a humane immigration system where people are treated fairly and we comply with our international obligations.

“Imagine if you’re told to do something on Tuesday, that in July is subsequently found to be illegal. That would be an appalling situation.

“I mean, if Priti Patel had any respect, not just for the desperate people who come to this country, but for the workers she employs, she would not ask a single one of them to be part of any deportation of any asylum seeker, until at least these cases are heard in court for the full legality judgment, which will be in July.”

The judge in Friday’s court hearing, Mr Justice Swift, refused to grant “interim relief” after an injunction application was made by some of the asylum seekers facing deportation to Rwanda.

The decision will not stop individual refugees from further legal challenges to their removal or a judicial review of the policy, which Swift said could take six weeks.

The PCS is involved in an appeal over the case on Monday. Serwotka said there needed to be a debate not only about the policy’s legality, but also its ethical basis.

“When people are fleeing from Iraq or Iran or Syria or Yemen and they come to this country, they are fleeing death, torture and persecution,” he said. “We should welcome people to this country and treat them civilly and assess their cases when they are here.

“We hope we win tomorrow in the court of appeal to stop the flight.”

The policy is intended to deter people from using unofficial routes such as crossing the Channel in small boats to enter the UK and claim asylum. The numbers of people doing so has not yet dropped, but Downing Streetsays the impact will only be seen when the policy is fully up and running.

Critics argue that the relatively small numbers who could be sent to Rwanda will not put people off, and that the fact people with children will not be deported will simply provide an incentive to take minors on the boats.

The Northern Ireland secretary, Brandon Lewis, also defended the policy on Ridge’s show, while refusing to comment on what he called “rumoured” criticism from Prince Charles, who reportedly called the plan “appalling”.

Asked if he was personally comfortable with the policy, Lewis said: “Yes, I am, actually.

“The reality is this is a policy that is going to deliver to ensure that modern slavery and these people smugglers know that their criminal methods will be broken down.”