A new immigration bill in the UK that aims to stop migrants crossing the Channel illegally on small boats from France is "incompatible" with Britain's international obligations, Europe's top rights body said on Monday.

In a letter addressed to both chambers of the UK parliament, the Council of Europe (COE) Commissioner for Human Rights, Dunja Mijatovic, urged lawmakers to vote against the bill proposed by British Prime Minister Rishi Sunak's Conservative government.

It is now "essential" that lawmakers "prevent legislation that is incompatible with the UK's international obligations being passed," she said in the letter.

The draft law intends to outlaw asylum claims by all illegal arrivals, while anyone who is deported after making the dangerous journey from France would be banned from re-entering the UK.

Migrants would be returned to their home country or sent on to a "safe" destination such as Rwanda, under a hotly contested partnership agreed by London.

"By effectively preventing people arriving irregularly from having their asylum claims assessed, the bill would strip away one of the essential building blocks of the protection system," said Mijatovic.

If passed, the bill would "add to the already significant regression of the protection of the human rights" of refugees, asylum seekers and migrants in the UK and "provide an incentive" to other states in Europe and beyond to adopt similar measures, she said.

Even after Brexit, UK remains a member of the COE, which is a separate entity from the European Union. 

Some lawmakers in Sunak's Conservative Party want the legislation to go even further, with several calling on the government to take Britain out of the the European Convention of Human Rights (ECHR) to drive through tighter controls.

Meanwhile, other Conservatives and opposition politicians want the government to offer more so-called safe routes, something Sunak has promised but only when the numbers arriving have fallen.

Sunak has said he believes Britain could introduce the new legislation while remaining compliant with the ECHR and international obligations.

(With input from Reuters and AFP)