Emmanuel Macron is to become the next president of France after winning a projected 65% of the votes, exit polls show.
French polling agencies showed the centrist candidate has triumphed over the far-right's Marine Le Pen.
The National Front leader is forecast to have won just 35% of the votes.
Theresa May was quick to congratulate Mr Macron on his victory.
A Downing Street spokesman said: "The Prime Minister warmly congratulates President-elect Macron on his election success.
"France is one of our closest allies and we look forward to working with the new President on a wide range of shared priorities."
Ms Le Pen, who had hoped to capitalise on the disillusionment of voters in France, said she had called Mr Macron to congratulate him on his victory just minutes after the polls came in.
French Prime Minister Bernard Cazeneuve said the vote was a rejection of the "deadly project of the extreme right".
Crowds outside the Louvre in Paris, where a party was being held for Mr Macron, waved flags as news of his victory came in.
A former investment banker, Mr Macron became economy minister in 2014 but resigned two years later to set up his own movement, En Marche!.
He will take France's top job without having previously been elected to public office.
French President Francois Hollande said the results showed that the majority of voters supported the European Union.
Ms Le Pen had wanted to pull the country out of the bloc and euro in a bid to restore French sovereignty - views which put her at odds with her pro-European opponent.
Mr Hollande said he had called Mr Macron to congratulate him on his victory.