Nigel Farage announced a "new entente cordiale" with France's right-wing National Rally party as he pledged to work together to stop small boats crossing the Channel.
The deal, signed by Farage and National Rally President Jordan Bardella at the Reform conference on Friday, commits the two parties to co-operate on returning migrants to France.
The Reform UK leader referenced the early 20th century treaties between the UK and France to signify the close relations between his party and Bardella's if both win their next national elections.
France is due to hold its next presidential election in April. National Rally has led in polling since March last year.
Bardella's predecessor, Marine le Pen, is due to be the party's candidate for president of France. She previously led the National Front party.
Farage told the Reform conference: "We today are going to begin a genuine new entente cordiale between our parties and our countries."
He added: "We are about to sign a memorandum of understanding between our two parties, and we hope and believe our next two governments, where we reaffirm our commitment to stopping illegal immigration, where France say they'll do their utmost to stop all departures and maybe this time, with him as prime minister, they will actually mean it."
Addressing the event in English, Bardella said: "The great challenge is one we all know; taking back control of immigration. In recent years, [French President] Emmanuel Macron has failed to protect our country.
"For him mass migration is not a problem, it's a project. France must now regain the trust of its own people, but also the trust of its neighbours and partners. France must take back control, and France must finally keep its word.
"Dear friends here in Birmingham, I make you this promise. If the French people bring us to power in 2027 in a few months, with the Rassemblement National [National Rally] and Marine le Pen, our government will do everything in its power to ensure that France is no longer the gateway for illegal migration into the United Kingdom."
France and the UK already have a £501-million ($677-million) deal in place, signed by Keir Starmer's government in April, designed to stop launches from the French coast.
Channel crossings are down 40% compared to last year. The number of crossings this summer are at their lowest level since 2019. However, thousands are still arriving – with larger boats being used to transport migrants over the summer.
Prime Minister Andy Burnham and Macron discussed the issue during the latter's visit on Thursday. A boost in the number of police officers tackling "mega-dinghies" was announced in advance.
Bardella told the Reform conference: "The destination of illegal migrants entering France should not be the United Kingdom, they should be returned to their countries of origin."
He continued: "We must put an end to these deadly routes, we must break the smuggler business model and together we must take control of our borders.
"If we win the election, our government's immigration policy will be simple. You do not enter France simply because you have decided to. You enter France because France has allowed you to.
"We will restore control and protection at our borders and we will send foreign criminals back to their countries."