UN High Commissioner for Refugees Filippo Grandi (Photo: AFP/ VNA)

More than 45,000 migrants arrived in Britain from continental Europe in small boats last year. The legislation to deter such crossings includes measures to transfer all irregular arrivals to "safe" third countries like Rwanda. None have been transferred so far.

Türk and Grandi said the law contained no guarantees that asylum seekers could access protection in Rwanda. Unaccompanied children also fall within the scope of the law. Without viable removal arrangements, thousands can be expected to remain in Britain indefinitely in precarious legal situations, the two UN officials said.

The bill, which has been passed by Parliament, now awaits the formality of being signed into law by King Charles III.