Nigel Farage is set to announce that a Reform UK Government would deport as many as 400,000 asylum seekers who broke into Britain illegally.
The Reform chief is set to commit to an immediate examination of all asylum decisions made over the past five years.
The review would track down anyone who entered illegally or overstayed their visa and subsequently claimed asylum - which home affairs spokesman Zia Yusuf pledged would "reverse the invasion of Britain".
Home Office staff under a Reform Government would be tasked with investigating three key factors under the proposed scheme.
Officials would determine whether asylum seekers crossed the Channel on small boats, arrived hidden in lorries or aircraft, or overstayed their visas before lodging their claims.
They would also assess whether an individual's country of origin is now considered safe by Government standards.
Should any of these conditions be met, the refugee and their family members would lose their permission to stay in Britain.
Those affected would be allocated an unspecified timeframe to depart voluntarily.
Mr Farage will commit to an immediate examination of all asylum decisions made over the past five years
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The overwhelming majority of those seeking asylum reached Britain via small boat, vehicle, or by switching to refugee claims after entering on work, study or tourist visas.
Anyone refusing to leave of their own accord would face detention and forced removal through Reform's proposed Deportation Command.
This new body would bear responsibility for expelling migrants who entered illegally.
The party estimates that over 400,000 people would fall within the scope of this retrospective policy.
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Reform UK estimates that over 400,000 migrants would fall within the scope of the policy
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This number comes on top of 600,000 unsuccessful asylum applicants whom Reform UK has already pledged to remove.
Stripping status from those who have already had their asylum granted marks a radical doubling-down on the party's earlier position of simply blocking illegal arrivals from making claims.
Mr Yusuf said: "For years, Tory and Labour governments have presided over an invasion of Britain.
"They have effectively operated an open borders policy. Instead of upholding the law, they have rewarded those who broke it by entering Britain illegally."
Zia Yusuf accused successive governments of overseeing 'an invasion of Britain'
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He added: "Reform will reverse this. Today we announce that a Reform government will review the previous five years of asylum grants, and anyone who broke into the country illegally or overstayed on another visa will be stripped of their status and deported. We will do what it takes to restore justice in Britain."
But the party is bracing for backlash, with the proposals expected to draw fury from NGOs and refugee organisations.
In the face of potential court challenges, Reform UK is also pledging to change the law and exit international refugee agreements.
The party has committed to leaving the controversial European Convention on Human Rights (ECHR) and abandoning the 1951 Refugee Convention.
A proposed Illegal Migration (Mass Deportation) Act would legally compel the Home Secretary to expel any migrant lacking residence rights.
Among the more than 100,000 asylum applications submitted last year, roughly 41 per cent arrived on small boats.
An additional 11 per cent came hidden in lorries or without proper documentation.
Approximately 39 per cent made their claims after arriving legitimately on work, study or holiday visas.
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