British Prime Minister Rishi Sunak has made tackling the arrival of small boats across the English Channel one of his priorities, hoping his move to get tough on illegal migrants will win over voters before an election expected next year.

But the Council of Europe's Dunja Mijatović said the Illegal Migration Bill would strip away an essential part of the protection system by preventing people arriving irregularly in Britain from having their asylum claims assessed.

"It is essential that parliamentarians prevent legislation that is incompatible with the United Kingdom's international obligations being passed," Mijatović wrote in a letter to both houses of parliament, ahead of the bill being debated on Monday.

"Passing the bill would add to the already significant regression in the protection of refugees, asylum seekers and migrants in the UK in the last few years."

The government says the bill is needed to limit the arrival of migrants on small boats mostly from France and break the trafficking rings who prey on them.

But 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 European Convention of Human Rights (ECHR) to drive through tighter controls.

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.

"It is a tough piece of legislation, the likes of which we've never seen," he told reporters on Monday.

"It's important that it's effective, which it will be, but it's also important that we abide by our international obligations. This is a country and a government that does follow the law."