"We are confident that we are complying with the law, domestic and international," she told the BBC.
Under the government's plans, almost all asylum seekers who reach Britain in small boats will be detained without bail before they are deported to their home country or, if this is not safe, another destination such as Rwanda.
They will also lose the right to challenge their deportation while in Britain, and once deported will be automatically banned from returning.
Last year, a record 45,000 people came to Britain in small boats across the Channel, mainly from France.
If everyone who arrived in small boats last year were detained, this would be equal to about half of Britain's total prison population.