Ex-Home Secretary Suella Braverman says the government's new Rwanda law "won't work".
"The reality is, and the sorry truth is, it won't work," she tells the BBC. "It will not stop the boats".
She urges Rishi Sunak to change course - otherwise, she says, the new Rwanda law "will fail".
Braverman was speaking after Immigration Minister Robert Jenrick resigned - saying the new law does not go far enough.
The government wants to send asylum seekers on a one-way ticket to Rwanda - but the policy has been blocked in the courts.
A new law aims to make it harder for courts to block the policy, but Jenrick and now Braverman say it's not tough enough.
The PM described Jenrick's resignation as "disappointing", and "based on a fundamental misunderstanding of the situation".
Jenrick has previously suggested the government could quit the European Convention on Human Rights (EHCR) - instead, the bill allows ministers to disregard parts of UK human rights law.
