Francois Hollande has warned there will be "consequences" for the way France seeks to control immigration into Britain if the UK votes to leave the European Union.
Following a UK-France summit in Amiens, the French president said Britain could not escape the fact that relations between the two countries would change in the event of a vote for Brexit.
While he stopped short of repeating a warning by the French finance minister that UK border controls in Calais could be withdrawn - potentially opening the door for many more migrants to cross the Channel - he left little doubt the issue would be on the table.
David Cameron dismissed suggestions that the French intervention was part of a giant "David Icke-style" conspiracy to "stitch-up" the result of the referendum on Britain's EU membership as "nonsense".
The Prime Minister stressed, however, that it was important that voters listened to what friendly countries such as France were saying when they came to decide.
Mr Hollande denied that he was trying to "scare" UK voters, insisting such tactics would be counterproductive.
However, he made clear that it would not just be immigration issues that would be affected - pointing also to the European single market and the financial services sector.
He acknowledged, nevertheless, it was the situation at Calais - where hundreds of migrants are camped in the Jungle while they try to find a way across the Channel - that was the most sensitive issue.
"I don't want to scare you but I just want to say the truth. There will be consequences in many areas," he said.
"There will be consequences, especially in the way we handle the situation we have just mentioned in terms of immigration. There will be consequences, there is no solution where there is no consequences."