Hollande echoed his economy minister in speculating that the focus of the two countries' migrant crisis could shift from Calais to the British side of the English Channel.
"I don't want to scare you, I just want to tell the truth, there will be consequences if the UK is to leave the EU," Hollande said at a news conference, standing next to British Prime Minister David Cameron. The two leaders had just attended a UK-France summit in Amiens in northern France Thursday.
"There will be consequences, especially in the way that we handle the situations that we just mentioned in terms of immigration," he said, an apparent reference made by Economy Minister Emmanuel Macron to British newspaper Financial Times ahead of the meeting.
The British Prime Minister announced Thursday £17 million ($24 million) in additional funding to help France address the situation at controversial Calais migrant camp known as the "Jungle."
The camp on the outskirts of the French city closest to the UK is a major transit point for thousands of migrants hoping to enter Britain illegally by smuggling themselves on trucks or other vehicles. Many migrants there are reluctant to leave and register in a French reception center as their preferred destination is the UK.
Cameron had earlier said
the notorious migrant camp may move to British shores if the UK voted to leave the EU in June's 'Brexit' referendum. The move would reflect the potential revision of an Anglo-French accord, known as the Le Touquet treaty, which allows the UK to station immigration officials on French soil.
"Brexit is the other face of the refugee crisis," Macron said, according to a transcript of his remarks
"The day this relationship unravels, migrants will no longer be in Calais," he said.
Hollande also alluded to other downsides for Britons aspiring to greater independence in Europe.
"There will be consequences in many areas: on the single market; on financial trade; the economic development of the two countries. It doesn't mean that everything will be destroyed, I don't want to give you a catastrophic scenario but there will be consequences, especially in terms of people as well."
Hollande sought to confirm the endurance of the two countries' "historic relations" and "friendship" but warned that a move to exit the bloc would be consequential.
"There is no solution where there is no consequences. There are consequences if the 'in' wins or if the 'out' wins, there will be consequences both ways. Every time the people speak out in a referendum, there are consequences," he said.
UK politicians who support the split were dismissive of discussion that the treaty would be revised, with London mayor and prominent "out" campaigner Boris Johnson
speaking pidgin French and denying that there would be a migrant processing center to replace Calais on British soil.
"I would say: 'Donnez-moi un break.' There's absolutely no reason why that treaty should be changed," the Guardian reported him saying
. "It was an intergovernmental treaty; it was the Le Touquet treaty. It was signed between the British government and the French government. It's not in the French interests to want to do that and it's just the usual flapping and scaremongering."
Tusk: Migrants stay away
European Council President Donald Tusk on Thursday warned opportunistic "economic migrants" -- those who are not legitimate refugees but are seeking better living conditions -- not to come to Europe, saying their efforts would be "all for nothing."
"I want to appeal to all potential illegal economic migrants wherever you are from: Do not come to Europe. Do not believe the smugglers," Tusk told reporters in Athens, Greece.