The United Nations' children's agency on Friday joined critics of a proposed British law aimed at stopping migrants arriving by small boat, saying it was "deeply concerned" about its impact on minors.

Jon Sparkes, head of United Nations Children's Fund in the UK, said the bill could deny children and families the chance to seek safety in the UK.

"For almost all children fleeing conflict and persecution there is no safe and legal route into the UK," Sparkes said.

The bill, presented on Tuesday by Prime Minister Rishi Sunak’s Conservative government, plans to ban people who have arrived in the country illegally from seeking asylum.

Instead they will be detained and sent to a third country deemed safe.

Exceptions would be made for minors, but only if they were unaccompanied.

Sparkes said the bill was published without an assessment of its impact on children, leaving many critical questions unanswered.

"It is not clear how this bill will be compatible with existing UK government duties to act in the best interests of the child, and it is questionable whether the removal of a child to a third country, following a perilous journey to the UK, could ever be in their best interest," he said.

He called on the British government "to urgently clarify how it intends to ensure the safety and well-being of children with this bill, and how it will respect its obligations regarding the defence of children's rights".

More than 45,000 migrants arrived in the UK last year by crossing the English Channel on small boats.

According to official figures, 17 percent of people who took the Channel route to the UK since 2018 are minors.

"UNICEF UK maintains that the creation of safe and legal routes must be part of any compassionate and effective response to reducing the use of unsafe routes," Sparkes added.

Other UN agencies have also criticised the planned new law.

The UN High Commissioner for Human Rights said it "would be at odds with the country's obligations" on human rights and refugee law -- a claim denied by London.

Illegal immigration was on the agenda on Friday during Sunak's visit to Paris.