The U.K. could send tens of thousands of asylum-seekers to the East African country of Rwanda, Prime Minister Boris Johnson said Thursday, aiming to break people-smuggling networks and stem the flow of migrants across the English Channel.

Concerns over immigration were a big factor in the 2016 Brexit vote, and Johnson has been under pressure to deliver on his promise to "take back control" of the country's borders. But his plan drew swift criticism from opponents of his Conservative Party and from charities.

"We must ensure that the only route to asylum in the U.K. is a safe and legal one," Johnson said in a speech in Kent, southeast England, where thousands of asylum-seekers in small boats landed on Channel beaches last year. "Those who try to jump the queue or abuse our systems will find no automatic path to set them up in our country, but rather be swiftly and humanely removed to a safe third country or their country of origin."