"There will inevitably be a price and a cost for Britain, it's the choice they made," Hollande said as he arrived at a Brussels summit.
"We must not be punitive, but at the same time it's clear that Europe knows how to defend its interests, and that Britain will have a less good position outside the EU than in the EU."
Hollande, who is entering his last days as French president, dismissed suggestions that British Prime Minister Theresa May could strengthen her negotiating hand by winning a big mandate in elections that she has called for June 8.
"I can understand the electoral argument but it will not influence the EU. The EU's principles and the objectives are already fixed, these will be the lines chosen by negotiators."
Luxembourg Prime Minister Xavier Bettel also ruled out an advantage for May from a big election win.
"We are very united, you seem surprised, but it's a fact."
The EU's chief Brexit negotiator Michel Barnier meanwhile said it was also in Britain's interests for the EU to be unified, as it would boost the chances of a Brexit deal.
"This extraordinary meeting shows the unity of the 27 on a clear line, but this unity is not directed against Britain, I think that it is also in its interest," he said.
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