The remarks from European Commission president Jean-Claude Junker come as Britons prepare to decide whether to stay or leave the 28-state bloc.
Leaders at home and abroad are voicing their concern about the possible consequences of a vote to sever ties.
European Commission president Jean-Claude Junker has made it clear that, whatever Britain elects to do after its referendum, the decision would be final.
He says, if Britain remains part of the European Union, there will be no new membership terms to negotiate on top of the special status it secured earlier this year.
"The British policymakers and the British voters have to know that there will be no kind of any renegotiation. We have concluded a deal with the Prime Minister. He got the maximum he could receive, and we gave the maximum we could give, so there will be no kind of renegotiation. Out is out."
They are stern words of warning as political advocates on both sides of the argument make a final push to get their messages across.
More than 46 million people are eligible to vote in the referendum, and almost one in 10 are believed to still be undecided.
Several late-stage polls have suggested the numbers are too close to predict the outcome with certainty.
And although the vote is not legally binding, the government is expected to abide by the result.
Leading the charge for Britain to exit the European Union is the United Kingdom Independence Party, or UKIP, and its leader, Nigel Farage.
"Vote with your heart. Vote with your soul. Vote with pride in this country and its people, and, together, we can make tomorrow our independence day, a big day in our national history."
But on the eve of the referendum, Prime Minister David Cameron, too, made a final plea.
"This vote, if we leave, is irreversible. If you jump out of the aeroplane, you cannot clamber back through the cockpit hatch."
And Mr Cameron portrayed the vote as a historic moment.
"That is why anyone in any doubt should be voting 'Remain' tomorrow. This is a choice not just for this parliament, the next five years, the next 10 years. It is the choice of a lifetime for people in our country, particularly young people."
Opposition leader Jeremy Corbyn called on followers to protect universal healthcare and employment by voting to stay.
"I urge Labour supporters all over the country to think very carefully about this. Do what's best for our people. Vote for jobs, vote for rights at work, vote for our NHS. Vote to remain in the European Union."
World leaders have also voiced strong opinions in the final hours before the vote.
French president Francois Hollande says there can be no going back if Britain decides to leave the European Union.
"There would be a very serious risk for Britain to lose its access to the single market and everything that goes with the European Economic Area. It's France's position that, if exit from the Union is chosen, then all the conclusions must be drawn, and that would be irreversible."
The leader of the economic and political bloc's largest economy, German chancellor Angela Merkel, has reiterated that view.
"All aspects of the EU's domestic market, and the benefits shared by all EU members, would no longer be available for Britain if it votes to leave. All forms of negotiations conducted by the EU on behalf of all its 28 members will become the negotiations between the 27 members and a third party. I can hardly imagine what benefit would come out of this."
International banking bodies, meanwhile, are watching the developments closely.
The World Bank downgraded its global growth forecast in the months before the referendum.
The International Monetary Fund warned leaving the European Union could trigger a recession in Britain as early as next year.
United States Federal Reserve chairwoman Janet Yellen indicated the Reserve, too, is thinking about the economic implications.
"Investor perceptions of, and appetite for, risk can change abruptly. One development that could shift investor sentiment is the upcoming referendum in the United Kingdom. A UK vote to exit the European Union could have significant economic repercussions. For all of these reasons, the committee is closely monitoring global economic, financial developments and their implications for domestic activity, labour markets and inflation."
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