Reform UK has proposed using a major military operation to intercept asylum seekers crossing the English Channel in small boats if the party wins power.
Party leader Nigel Farage said the plan, called "Operation Fortress", would involve what he described as the largest military operation in the Channel since the Second World War.
The policy would aim to intercept boats carrying asylum seekers and return them to France, with Reform saying it would make the UK's borders effectively "impenetrable".
"Under a Reform government, within a fortnight, there would be no more boats," Mr Farage told reporters.
The announcement comes as the party faces growing scrutiny over its finances and a decline in support in national opinion polls.
Reform had led the polls for more than a year before recent surveys showed it falling behind other parties. The change followed a series of reports concerning gifts and donations involving senior party figures.
The Labour Party accused Reform of reviving existing immigration proposals to divert attention from questions about its funding.
The feasibility of the plan has also been questioned.
Andrew Fox, a former paratrooper and senior associate fellow at the Henry Jackson Society think tank, said the proposal overlooked significant legal and operational obstacles.
He said the UK could not send warships into French territorial waters, seize vessels or land passengers in Calais without the agreement of the French authorities.
"This is the gap at the centre of the policy," he said.
Reform has pointed to migration pressures elsewhere in Europe, including the recent situation in Spain's North African enclave of Ceuta, as evidence of the need for a tougher approach to immigration.
The Channel has become a major route for people attempting to reach the UK in small boats, with successive governments struggling to reduce crossings.
Prime Minister Andy Burnham pledged on Sunday to take "relentless" action to tackle the crossings after figures showed that more than 2,000 people had arrived in Britain since he became prime minister two weeks ago.
Reform's proposal is likely to intensify debate over how far the UK should go in using military resources to address irregular migration, as well as the legal and diplomatic implications of returning asylum seekers to France. Photo by Gage Skidmore from Peoria, AZ, United States of America, Wikimedia commons.
