The leaders of Britain's Reform UK party and France's National Rally announced a pact on Friday that they said would enable Britain to return "asylum seekers" arriving in small boats to France if their parties win power.
For the past decade, both countries have struggled to curb the flow of "asylum seekers" travelling from France to Britain, often in overcrowded, inflatable boats.
Speaking at Reform's annual conference in the central English city of Birmingham, RN party chief Jordan Bardella said a National Rally government would tighten border controls, return illegal migrants to their home countries and stop boats crossing to Britain.
"I make you this promise, if the French people (give us the presidency) our government will do everything in its power to make sure that France is no longer the gateway for illegal migration into Britain," he said to a standing ovation.
Reform leader Nigel Farage and Bardella later appeared on stage together where they signed a memorandum of understanding.
The agreement said "asylum seekers" arriving in Britain from France would be detained and returned to France, which would repatriate them to their countries of origin.
The pact said both countries would establish a mechanism for sharing border security costs.
Opinion polls predict that National Rally's Marine Le Pen will win the first round of the next presidential elections next April by a large margin, and will also be well-placed to win the run-off in May.
Reform, which topped polls for almost 16 consecutive months until the summer, is neck-and-neck with the governing and deeply unpopular Labour Party. The next national election in Britain must be held by 2029 at the latest.
Last month, Reform said that if it got elected, it would stop "asylum seekers" in what it described as the largest military operation in the channel since World War Two, including plans to return "asylum seekers" to France even without permission.
The French Government rejected the plans saying it would be a violation of its sovereignty.
More than 16,000 people have been detected arriving in Britain in small boats since the start of the year, a decline of more than 40 per cent. But the number of "asylum seekers" arriving on each boat was increasing as people smugglers change their tactics.
Earlier this week, British Prime Minister Andy Burnham and French President Emmanuel Macron announced that extra police would be deployed on beaches to deter small boat crossings.
Farage has previously sought to distance himself from the conservative movement in France, but while he said he had been "nervous" of Marine Le Pen's father, former presidential candidate Jean-Marie, he praised her for changing her party.
"What she has done with the party...getting rid of those extreme elements, putting it into the mainstream of French national debate, is a massive achievement," he said.
(Reporting by Andrew MacAskill and Alistair Smout in Birmingham and Sam Tabahriti in London, editing by Sarah Young, Elizabeth Piper)
