Nigel Farage has said his party would use the Navy to intercept and forcibly return small boats crossing the English Channel.
The Reform UK leader insisted “Operation Fortress”, which would see migrants transported back to France or Belgium, would comply with international law.
Mr Farage described the scheme as a humanitarian response.
But the French government hit back at the plan saying it would be a “violation of French sovereignty”.
“Operation Fortress will be a combined military operation,” Mr Farage told a press conference in London on Monday.
“It will save lives, it will help the French in fulfilling their job of stopping the boats and it will save a huge amount of money.
“We are very confident that our policy of securely picking people up and safely taking them back to France - we are entirely confident that that falls within other conventions of international law.”
But a spokesperson for the French interior ministry said in a statement: "This would be a violation of French sovereignty as well as a violation of maritime law and international law.
"Separately, as part of the measures put in place with the UK and the arrangements deployed at the local level, the interior ministry states that the number of crossings is down in 2026."
It comes as Home Office figures show 1,176 people have made the perilous journey from France to the UK in the last seven days alone.
Despite a recent flurry in arrivals, the number of migrants who have reached the UK after crossing the English Channel so far in 2026 is more than 40 per cent lower than at this point last year.
The provisional total for 2026 so far is 14,526 people, 43 per cent lower than the number recorded by this time last year (25,436) and down 14 per cent on the same period in 2024 (16,903).
Mr Farage added: “When you think about it, what is the Royal Navy for?
“If the Royal Navy is not there to protect this country from invasion, then, frankly, I can’t see what its purpose is.”
Reform said it would set up a joint military command with units from the Royal Navy, Royal Marines, RAF and British Army as well as surveillance.
It said the navy’s River-class offshore patrol vessels would be used for interception.
The ships’ current responsibilities include monitoring Russian naval activity around British waters.
Reform said the plan would not involve the use of so-called “pushbacks” by towing or barging their dinghies away from British waters.
The navy would intercept any unauthorised boats and detain their occupants, giving them water, food and a medical assessment before returning them to France or Belgium under escort on navy vessels, it said.
Andy Burnham over the weekend vowed that his government would be "relentless" in bringing down the number of people arriving in Britain on small boats.
The Prime Minister insisted progress was being made and the Government was "changing the whole approach".
The number of enforcement officers targeting people-smuggling gangs had doubled on both sides of the Channel, he said.
"People want to see this issue addressed," he said in Dover.
"We also need to build a system that offers safe routes for people as well, because that also takes away the selling point of the gangs."
Downing Street said that when the Royal Navy had primacy for small boat crossings in 2022, there was no reduction during that period.
Asked about the proposals, the Prime Minister’s official spokesman said on Monday: “The Royal Navy had primacy for small boat crossings under the previous government in 2022. I’d remind you there was no reduction in small boat crossings in that period.
“The Royal Navy is already tackling many threats, including monitoring any Russian warships operating near our shores, protecting our waters and our critical national infrastructure.
“Border patrol officers have significant capabilities and skills that make them the best-suited candidate to lead small boats operations in the Channel.
“That’s why we’re driving down the number of crossings. It’s why they’re down almost 50 per cent compared to last year and why almost 60 per cent of attempted crossings have been stopped since this Government came to power.”
Reform said the situation in the north African Spanish enclave of Ceuta, where some 60,000 migrants breached the border to enter Spanish territory earlier this week, showed the “urgency” needed.
Most of the migrants in Ceuta had walked back to neighbouring Morocco by Sunday.
Last Wednesday marked the busiest day for small boat crossings across the English Channel so far this year, with 752 migrants arriving in the UK.
Shadow home secretary Chris Philp said: “Reform UK have no detailed plan nor funding to back up their latest gimmick.
“They can’t explain how they will properly fund the military and they can’t explain how they would disembark illegal immigrants in France without French co-operation.”
