British authorities have started to detain migrants in preparation for them to be sent to Rwanda in the next nine to 11 weeks, the government said on Wednesday, laying the groundwork for Prime Minister Rishi Sunak's flagship immigration policy.

Parliament last month approved a law that paves the way for sending asylum seekers to Rwanda if they arrive in Britain without permission. Sunak, who is expected to call an election later this year in which illegal migration is likely to feature prominently, wants the first flights to take off in July.

More than 7,500 migrants have arrived in England on small boats from France so far this year. The government says the new law will deter people from making the perilous trip across the Channel. Five people died trying to make the crossing last week.