Britain's Prince Charles has called the government's plan to send asylum-seekers to Rwanda "appalling", a report said Saturday after it cleared a legal challenge.

The UK government intends to fly the first planeload of 31 claimants to Rwanda on Tuesday -- shortly before Charles is due to represent his mother Queen Elizabeth II at a Commonwealth summit in Kigali.

Prime Minister Boris Johnson -- who is also set to attend the summit -- welcomed his government's victory in Friday's High Court hearing, although an appeal is due to be heard on Monday. 

"We cannot allow people traffickers to put lives at risk and our world leading partnership will help break the business model of these ruthless criminals," Johnson tweeted.

Charles, however, joined others including senior Christian clerics in denouncing the plan, and fears the issue could overshadow the Commonwealth summit on June 24-25, The Times reported.

"He said he was more than disappointed at the policy," the newspaper quoted an unidentified source as saying. 

"He said he thinks the government's whole approach is appalling. It was clear he was not impressed with the government's direction of travel," the source added.

A spokesman for Charles declined to comment on private conversations, "except to restate that he remains politically neutral".

"Matters of policy are decisions for government," the spokesman added.

The government says the one-way flights will deter asylum claimants from entering Britain by illegal routes, and offer those who do try a new life in Rwanda instead.

More than 10,000 migrants have made the perilous sea journey from France to Britain so far this year, a huge increase on prior years.