The Government’s flagship Illegal Migration Bill passed through Parliament but has been denounced by the United Nations while plans for more vessels to house asylum seekers have run into difficulties.

The legislation, central to Prime Minister Rishi Sunak’s pledge to “stop the boats” crossing the Channel, breaks the UK’s obligations under international law, the UN said in an unusually critical statement on Tuesday.

It came as the Bibby Stockholm accommodation barge was met by protesters as it arrived into Dorset’s Portland Port, while two further cruise ships set to house migrants have reportedly been unable to find a berth.

The Bibby barge’s arrival came after a night of drama in which the Tory frontbench saw off changes being sought by peers to the Illegal Migration Bill, including modern slavery protections and child detention limits.

The cessation of the stand-off between the unelected chamber and MPs paves the way for the Bill to receive royal assent and become law.

The reforms will prevent people from claiming asylum in the UK if they arrive through unauthorised means.

The Government also hopes the changes will ensure detained people are promptly removed, either to their home country or a third country such as Rwanda, which is currently the subject of a legal challenge.

The Bill encountered fierce opposition in the upper chamber, while UN human rights chief Volker Turk and UN high commissioner for refugees Filippo Grandi led national and international outrage at the plans.

This new legislation significantly erodes the legal framework that has protected so many, exposing refugees to grave risks in breach of international law

In a joint statement, they warned the Bill “will have profound consequences for people in need of international protection”.

“This new legislation significantly erodes the legal framework that has protected so many, exposing refugees to grave risks in breach of international law,” Mr Grandi said.

Mr Turk said: “Carrying out removals under these circumstances is contrary to prohibitions of refoulement and collective expulsions, rights to due process, to family and private life, and the principle of best interests of children concerned.”

They said the legislation will expose people to the danger of “detention and destitution” and put at risk “their rights to health, an adequate standard of living, and to work”.

Downing Street defended the Bill, with the Prime Minister’s official spokesman saying: “We are confident we are acting within international law.”

The Government’s plans to use further vessels, alongside the Bibby Stockholm, to accommodate migrants are in trouble as two giant cruise ships were turned down by two prospective berthing sites, according to Sky News.

Mr Sunak last month announced two more vessels would be used for migrant accommodation as part of efforts to reduce the £6 million daily cost of hotel accommodation while people await a decision on their status.

No 10 did not deny Sky’s report, which said plans for one of the boats to house migrants near Liverpool were scrapped after being declined by the port operator while another vessel was refused docking near Edinburgh.

The Prime Minister’s spokesman told reporters: “We are continuing discussions both in terms of those who can provide additional accommodation facilities and sites where it can be housed.

“That will continue.

“We want to open more.”

Around 50 asylum seekers will board the Bibby Stockholm from next week, the Home Office said, with the numbers set to rise to 500 over the next few months.

The Tory MP for West Dorset raised safety concerns relating to the overcapacity of the barge, which was originally designed for 250 people.

Chris Loder said it “cannot be deemed safe as no risk assessment can be provided” as he urged Home Secretary Suella Braverman to stop the use of the Bibby or to carry out the necessary risk assessments.

The barge arrived in Dorset on Tuesday a month behind schedule.

It left Falmouth, Cornwall, a day earlier after undergoing work to prepare it for its new role.

Locals have raised concerns about the Portland site being used to house asylum seekers and a band of placard-waving protesters gathered at the port’s gate to meet the barge’s arrival.

Resident Lorraine Beckett said: “It’s not the right place to be homing all of these people and it’s not right for Portland because we do not have the infrastructure on the island for the extra people coming in so that’s why I feel strongly by it.”

Downing Street defended the use of barges to house migrants, insisting it was a cheaper alternative to housing them in hotels.

Mr Sunak’s spokesman said: “It’s undergoing final inspections upon arrival.

“That’s the last part of the process ahead of the first group of asylum seekers moving into the vessel later this month.”