Rishi Sunak said migrants arriving in the UK illegally will be removed “within weeks” amid a growing backlash against the Government’s planned new laws to curb Channel crossings.

The Prime Minister also confirmed the Illegal Migration Bill – to stop people claiming asylum in the UK if they arrive through unauthorised means – will apply “retrospectively” if passed.

Unveiling the plans in the Commons earlier, Home Secretary Suella Braverman said asylum seekers arriving illegally will be detained and face a lifetime ban on returning after they are removed.

They will never be allowed to settle in the country or gain citizenship.

But she also told MPs the Bill was unlikely to be compatible with the European Convention on Human Rights, igniting fresh criticism after campaigners said the proposed policy would be unworkable.

Speaking at a Downing Street press conference on Tuesday afternoon, Mr Sunak said: “People must know that if they come here illegally, it will result in their detention and swift removal.

“Once this happens, and they know it will happen, they will not come and the boats will stop.”

Rishi Sunak speaking a press conference on Tuesday evening

However criticism of the controversial crackdown was growing on Tuesday night.

The United Nations’ refugee agency urged MPs and peers to block Mr Sunak’s “profoundly” concerning plan to tackle small boat crossings.

The UNHCR said the Illegal Migration Bill amounted to an “asylum ban” which would prevent people fleeing war and persecution from seeking refuge in the UK.

“We urge the Government, and all MPs and peers, to reconsider the Bill and instead pursue more humane and practical policy solutions,” the agency said.

The legislation “would be a clear breach of the Refugee Convention and would undermine a longstanding, humanitarian tradition of which the British people are rightly proud”, the UN agency said.

The Government’s approach, outlined by the Home Secretary in Parliament on Tuesday, has been widely condemned by charities and human rights organisations.

Mr Sunak, who visited Dover in Kent earlier in the day to mark the announcement, said the laws will make it “clear that if you come here illegally, you can’t claim asylum”, adding: “You can’t benefit from our modern slavery protection, you can’t make serious human rights claims and you can’t stay.

“We will detain those who come here illegally and then remove them in weeks, either to their own country if it is safe to do so or to a safe third country like Rwanda.

“And once you are removed, you will be banned – as you are in America and Australia – from ever reentering our country.”

Describing the move as “tough” but “necessary and fair”, he said: “And this legislation will be retrospective. If you come on a small boat today, the measures in this Bill will apply to you.”

But he conceded the UK will be “constrained” in its ability to take in genuine refugees in the future if it fails in its efforts to stop the boats.

“Full control of our borders will allow us to decide who to help and to provide safe and legal routes to those most in need,” he said.

“I understand there will be debate about the toughness of these measures. All I can say is we’ve tried it every other way and it has not worked.”

Asked if he will have failed if he has not “stopped the boats” by the next general election, Mr Sunak said: “I wouldn’t be standing here if I didn’t think that I could deliver on this promise.”

Ms Braverman told MPs it would “betray” British voters not to tackle the “waves of illegal migrants breaching our border”.

Rishi Sunak said the Illegal Migration Bill would apply ‘retrospectively’

But Labour’s shadow home secretary, Yvette Cooper, branded the Bill a “con” and described the plans as “Groundhog Day” less than a year after reforms were brought into force under the Nationality and Borders Act.

The comments came in the wake of criticism from campaigners who said the proposed policy would be unworkable. The UN’s refugee agency, the UNHCR, said it is “profoundly concerned” by the Bill and that, if passed, it will amount to an “asylum ban”, making it a “clear breach of the Refugee Convention”.

Ms Braverman said the need for reform is “obvious and urgent” – the asylum system now costs the British taxpayer more than £3 billion a year and there are a record number of cases awaiting a decision.

The Bill allows migrants to be detained without bail or judicial review for 28 days, with the intention that they will be removed within that time.

It places a duty on the Home Secretary to remove illegal entrants and will “radically narrow the number of challenges and appeals that can suspend removal”.

Only children and those “unfit to fly or at a real risk of serious and irreversible harm” in the country they are due to be sent to will be able to delay their removal.

Any other claims will be heard “remotely” once they have been deported.

The Bill will also introduce an annual cap, to be decided by Parliament, on the number of refugees the UK will offer sanctuary to through safe and legal routes.

Officials indicated hopes of the Bill being passed by the end of the year, which could see it in force ahead of any anticipated 2024 election.

In a letter seen by the PA news agency, Ms Braverman told MPs the Government is “testing the limits” and remains “confident that this Bill is compatible with international law” – but there is a “more (than) 50% chance” it may not be compatible with the European Convention on Human Rights (ECHR).

Mr Sunak said “we’re up for the fight” against any legal challenges to the plans, adding: “But we’re confident that we’ll win.”

He said there is “absolutely nothing improper or unprecedented” about pursuing Bills with a warning they may not be compatible with the ECHR.

Mr Sunak has staked his premiership on curbing Channel crossings, among four other priorities, in the face of pressure to tackle the issue amid dire polling figures for the Tories and has insisted changing the law is crucial.

In 2022, a record 45,755 migrants arrived in the UK after crossing the Channel.

More than 3,000 have made the journey so far this year.

Home Office figures show 197 made the crossing on Monday – the first arrivals since February 24 – taking the total to date to 3,150.