The Illegal Migration Bill risks breaching international obligations to protect human rights and exposing people to serious harm, the Equality and Human Rights Commission said.
The body issued the warning ahead of the Commons report stage of the proposed legislation on Tuesday.
The Bill, which would change the law to make it clear people arriving in the UK illegally will not be able to remain in the country, has prompted criticism from opponents who have dismissed it as unworkable.
But right-wing Tory MPs said the legislation does not go far enough, with some calling for ministers to take the UK out of the European Convention on Human Rights (ECHR) to gain tighter border controls.
The Commission said it “remains seriously concerned that the Bill risks placing the UK in breach of its international legal obligations to protect human rights and exposing people to serious harm”, adding: “Provisions providing for the detention of children and pregnant women and removing protections for victims of trafficking and modern slavery are particularly worrying.
“Effective, rights-compliant action is needed to ensure that more lives are not lost on dangerous Channel crossings. We welcome the Government’s commitment to increase safe, regular routes to the UK for those in need of asylum and recommend these are brought forward alongside the Bill.”
The Refugee Council and Barnardo’s have estimated the Bill could lead to the detention of nearly 15,000 lone migrant children over the next three years.
The calculations by the two charities were based on there being the same number of Channel crossings as last year (45,755), when 5,242 asylum applications where made on behalf of unaccompanied children.
The warnings come as the battle over the legality of the Rwanda deal continues, with a four-day Court of Appeal hearing beginning on Monday.
The Government’s plan to send migrants to the east African nation – a policy ruled lawful by High Court judges – has so far been stalled by legal action and no flights have taken off.
More than 5,500 migrants have arrived in the UK after crossing the Channel this year, according to Government figures.
According to provisional Home Office data, almost a quarter of migrants who made the journey between January and March this year were Afghans.
Out of the 3,793 people who arrived in the UK after crossing the Channel between January and March, 909 were Afghans (24%), making this the most common nationality. This was followed by Indians (657, 18%). Just 29 Albanians made the crossing during this period.
Of the 45,755 arrivals in 2022, 43,794 had their nationality recorded and of this total, 28% were Albanian nationals and 20% were Afghan.
Officials said there is a “seasonal effect” on crossings due to the weather, with more generally taking place in better conditions.
In light of the figures, the Institute for Public Policy Research (IPPR) think tank warned the Bill could leave Afghans “abandoned by the Home Office”.
Marley Morris, the think tank’s associate director for migration, trade and communities, said the statistics “expose the muddled thinking at the heart of the Government’s new migration bill”, adding: “Afghans left stranded after the disastrously executed withdrawal in 2021 will almost always have a well-founded protection claim, but under the Government’s migration bill any arriving by small boat on or after 7 March will be refused asylum and the Home Secretary will have a duty to remove them.
“And without countries to send people to, thousands will be trapped in limbo in the UK – unable to be removed and unable to claim asylum.
“While the Government claims there are safe routes for Afghans, these have been plagued by delays and difficulties. Only 22 people were resettled under one of the key Afghan pathways in 2022.”
The data also shows the total asylum backlog had fallen slightly since February this year but still stood at just over 138,000 at the end of March.
Mr Morris said the figures suggest that while some progress is being made in reducing the list of older asylum claims awaiting an initial decision, new ones continue to enter the system so the total outstanding figure is “roughly stable”.
“The migration bill will simply create a new backlog of people trapped outside the asylum system and with no right to work or access mainstream benefits,” he added.