Nigel Farage has agreed a deal with France's hard-right National Rally (RN) party on returning migrants who attempt to cross the English Channel in small boats, should both parties win power.
It comes after Reform UK last month announced a plan to intercept boats and return migrants to French ports using the Royal Navy.
The proposed agreement with RN states that France would "ordinarily accept" those on board, and then repatriate them to their home country.
It was signed at Reform's annual conference in Birmingham by RN President Jordan Bardella, whose party is currently leading in opinion polls ahead of presidential elections in April next year.
In a speech in the main conference hall, ahead of the deal being unveiled, Bardella told delegates that an RN government would "do everything in its power to ensure that France is no longer the gateway for illegal migration into the United Kingdom".
Hailing Farage as the UK's next prime minister, Bardella pledged to work with a potential future Reform government to tackle illegal migration.
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The "memorandum of understanding" unveiled at the conference is a political document, but could be developed into a legal agreement between two future governments, if both parties win power.
It states: "The United Kingdom will intercept boats which have launched from French shores and entered British territorial waters and will return the persons on board to France.
"France will ordinarily accept these persons, who will be repatriated to their home country in accordance with the law in force in France concerning expulsion."
Under the proposals, illegal migrants who successfully arrive in the UK from France would be immediately detained and deported to their home country, rather than to France, under plans previously outlined by Reform.
Challenged during a news conference over whether the wording "ordinarily" meant some migrants would not be accepted by France, Farage said it was "legalese".
He argued the proposals would mean there was "no incentive" to pay a trafficker to cross the Channel, and "in very, very short order the business model is broken".
Farage said the "historic document" would "stop the boats forever".
