The Council of Europe (COE) Commissioner for Human Rights, Dunja Mijatovic, urged UK Parliament lawmakers to vote against the bill proposed by Prime Minister Rishi Sunak's right-wing 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.
UK interior minister Suella Braverman has acknowledged that the government "pushed the boundaries of international law" to draft the bill.
The Sunak government, which has been languishing in the polls, has been under domestic pressure to stop the large numbers crossing the Channel.
Even after Brexit, Britain remains a member of the COE, which is a separate entity from the European Union.
London must abide by the European Convention on Human Rights which is overseen by the ECHR.
Mijatovic said the bill would create "clear and direct tension" with the human rights standards established under the ECHR.
Provisions in the bill indicating that the domestic legislation should supersede the ECHR risk "creating divergence with the case law of the ECHR", she said.
There have been suggestions that the UK could walk out of the ECHR but the Sunak government has insisted this is not on the agenda and has been in talks with the court.
The United Nations High Commissioner for Human Rights, Volker Turk, said earlier this month he was "deeply concerned" by the legislation.
Europe Council Slams UK Anti-Migrant Bill
STRAUSBOURG - 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.
