The government said it will deport migrants to Rwanda in two weeks on 14 June.
The Home Office said it had begun issuing formal removal notices to migrants as the “final administrative step” in its partnership with the east African nation.
The home secretary said that there would still be attempts to delay the process.
Priti Patel said: “Our world-leading partnership with Rwanda is a key part of our strategy to overhaul the broken asylum system and break the evil people-smugglers’ business model.
“Today’s announcement is another critical step towards delivering that partnership and, while we know attempts will now be made to frustrate the process and delay removals, I will not be deterred and remain fully committed to delivering what the British public expect.”
The Home Office said an initial group of migrants had started to receive formal letters telling them they are being sent to Rwanda to “rebuilt their lives in safety”.
It said the policy is designed to break people-smuggling networks and stem the flow of migrants across the Channel.
People who receive the letters can challenge them in court.
The government has said those sent to Rwanda will be given support, including up to five years of training to help with integration, accommodation, and healthcare.
Rwanda map
Read more:
Why are migrants being sent to Rwanda and how will it work?
Asylum seekers ‘willing to go into hiding’ to avoid Rwanda plan
First glimpse inside the centre that could house Channel migrants
The plan, announced in April, has drawn criticisms from MPs inside and outside the Tory party as well as from many charities.
UN officials have said the move would violate the international Refugee Convention.