"I am deeply concerned at this legislation," Volker Turk said in a statement.
Turk, the UN high commissioner for human rights, joined the chorus of concerns voiced by the UN refugee agency and a string of rights groups.
The draft law "would allow the UK authorities to detain and remove all those who enter the UK using small boats to cross the Channel, ban their future re-entry to the UK and prohibit them from applying for UK citizenship", he said.
"Such a wholesale ban preventing people from seeking asylum and other forms of international protection in the UK would be at variance with the UK's obligations under international human rights and refugee law," he warned.
Britain's Conservative government intends to outlaw asylum claims by all illegal arrivals and transfer them elsewhere, such as Rwanda, in a bid to stop thousands of migrants from crossing the Channel on small boats.
Turk said the legislation also raised a number of rights concerns, for example by violating the right to an individual assessment, as well as bans on arbitrary immigration detention.
It could also run afoul of bans on collective expulsions and so-called refoulement, or returning someone to a country where they could face torture, cruel, inhumane or degrading treatment or punishment, he said.
"All people compelled to leave their country of origin to seek safety and dignity abroad are entitled to the full respect of their human rights, regardless of their migration status or mode of arrival," he said.
