Fifty migrants have been told they will be the first to be sent to Rwanda in east Africa as part of the government's controversial resettlement policy.
In an interview with the Daily Mail, Boris Johnson revealed the figure, saying he expected a lot of legal opposition but insisted the government would "dig in for the fight".
Under a new deal, people judged to have entered the UK illegally now face being flown to Rwanda for resettlement.
The policy has been widely criticised.
Among its critics are more than 160 charities, the Archbishop of Canterbury, opposition parties and senior Conservative Party backbenchers, including former Prime Minister Theresa May.