The legality of Priti Patel's plans to turn back migrant boats at sea has been called into question by peers, including senior lawyers and a former judge.
The Lords Justice and Home Affairs Committee has written to the Home Secretary, expressing 'concerns' over the legal basis for the so-called pushbacks, which form part of her proposed Nationality and Borders Bill.
The letter adds to 'growing concern both in and outside Parliament' over the policy proposed in a bid to curb Channel crossings, peers said.
It comes as the Bill is being considered by MPs in the Commons at report stage for a second day today, ahead of a third reading.
Among other measures, the proposed law will also crack down on illegal migration by handing fewer privileges to asylum seekers who arrive illegally while increasing prison penalties for traffickers.
But it is the proposal of new powers to turn around migrant boats which is causing the most concern.
Ms Patel insisted the plan has a 'legal basis' when questioned by the committee in October, despite concerns being repeatedly raised over its legality and effectiveness which prompted campaigners to threaten her with legal action.
However, the Home Office's permanent secretary, Matthew Rycroft, previously conceded that only a 'small proportion' of boats could be turned back.
The Lords Justice and Home Affairs Committee has written to Home Secretary Priti Patel (pictured), expressing 'concerns' over the legal basis for the so-called pushbacks of migrants boats, which form part of her proposed Nationality and Borders Bill
The committee said it 'fully' endorses a report published last week by another group of MPs and peers, which found the tactic could endanger lives and is likely to breach human rights laws (Pictured: Migrants are brought ashore onboard a RNLI Lifeboat, after having crossed the channel, in Dungeness, Britain, November 24, 2021)
The committee's Liberal Democrat chairwomman, Baroness Hamwee, a former solicitor, said: 'Statements, including from the Home Secretary, are that there is a legal basis for the policy of so-called 'turnarounds'. We question that.
'The so-called 'turnaround' policy would force fragile small boats crossing the Channel to turn back.
'It is hard to imagine a situation in which those in them would not be in increased danger or where captains would not be obliged to render assistance.
'Instead, the Home Secretary has set a policy of forcing them to turn around.
'Even if there is a domestic legal basis, if it were actually implemented, it would almost certainly contravene the UN Convention on the Law of the Sea.
'Policing borders should be done in full accordance with the principles of national and international law, and we look forward to full engagement with our questions.'
Labour members Baroness Chakrabarti, a barrister and former director of human rights group Liberty, and ex-home secretary Lord Blunkett, Conservative member and solicitor Baroness Shackleton of Belgravia, and retired Court of Appeal judge and crossbench peer Baroness Hallett also sit on the committee.
Its letter, which calls for a response from the Home Office by January 5, asks under what powers the tactics could be used as the law stands currently.
The committee said it 'fully' endorses a report published last week by another group of MPs and peers, which found the tactic could endanger lives and is likely to breach human rights laws.
The turnaround tactics are 'not the solution' and will 'do the opposite of what is required to save lives', the Joint Committee on Human Rights said.
Labour MPs will be 'siding with criminal people smugglers' if they fail to back crucial immigration reforms, Priti Patel warned last night
More than 100 people including babies woman and children pulled from four Dinghies being unloaded at Dover Port on Saturday
It described the proposed Bill as 'littered' with measures which are 'simply incompatible' with the UK's international obligations.
It comes after Ms Patel told Labour MPs last night that they would be 'siding with criminal people smugglers' if they failed to back her Nationality and Borders Bill.
The Home Secretary suggested not voting for it could endanger more migrants' lives during perilous Channel crossings.
The proposed Bill will crack down on illegal migration by handing fewer privileges to asylum seekers who arrive illegally. It will also increase prison penalties for traffickers and people who enter illegally.
In response, Shadow Home Secretary Yvette Cooper said the Bill was a 'sham' and was 'failing' to tackle the issues.
In a letter to Ms Cooper, the Home Secretary wrote: 'Labour has a clear choice. They can side with the criminal people smugglers or with the British people.
'Voting for the Nationality and Borders Bill is standing against the people smuggling gangs, introducing tough life sentences and speeding up the removal of those with no right to be in our country, including foreign national offenders.
'Labour must choose between supporting our plan to reduce illegal migration and putting the British people's concerns first or continuing to back open borders.'
The Bill will attempt to slash the so-called 'pull factors' which ministers believe attract migrants to the UK.
Its key measures include creating a two-tier asylum system which will hand fewer privileges to those who arrive in Britain by illegal routes - even if their asylum claim is eventually accepted - compared with those who come by legal routes.
It will increase the maximum penalty for illegal entry to the UK from six months' to four years' imprisonment, and introduce life sentences for people traffickers.
The Bill has also been strengthened even further with the Government bringing forward changes to increase the maximum penalty for overstaying a visa from six months to four years' imprisonment.
Miss Patel insists the measures in the proposed laws are 'firm but fair'.
The Bill also gives Border Force officers controversial new powers to stop and divert vessels carrying migrants.
It also aims to speed up the appeals process by giving asylum seekers only one opportunity to lodge a legal challenge.
The measures come after more than 26,000 migrants have reached Britain since the start of the year, compared with just 8,410 in the whole of 2020.
In the worst tragedy yet seen during the Channel crisis, 27 migrants died off Calais last month when their inflatable boat sank.