Rwanda had not provided sufficient safeguards to prove it is a "safe third country", senior judges ruled in a split decision.

It is likely ministers will challenge the ruling in the Supreme Court and insist the policy will deter small boat crossings.
...
The High Court had backed the government's policy at an earlier hearing but that decision was scrutinised by Appeal Court judges Lord Chief Justice Lord Burnett, Sir Geoffrey Vos and Lord Justice Underhill in this latest stage of the process.

While Lord Burnett sided with the UK government, the others concluded that the assurances from the Rwandan government were not "sufficient to ensure that there is no real risk that asylum seekers relocated under the Rwanda policy will be wrongly returned to countries where they face persecution or other inhumane treatment".

https://www.bbc.co.uk/news/uk-66051292