European Union leaders Friday called for Britain to initiate the process of exiting the European Union as soon as possible and not delay the historic move.

Donald Tusk, Jean-Claude Juncker and Martin Schulz, the presidents of the European Council, Commission and Parliament, respectively, along with Netherlands Prime Minister Mark Rutte, whose country currently holds the EU's rotating presidency, said that they expected the U.K. to act on the result of the referendum “as soon as possible, however painful that process may be.”

Any delay, they argued, would “unnecessarily prolong uncertainty,” adding that there would be no renegotiation of the deal British Prime Minister David Cameron negotiated with EU leaders earlier this year.

Tusk also cautioned against “hysterical reactions,” to the referendum result, which is the first time a country has chosen to leave the bloc.

“We are determined to keep our unity as 27,” he said, referring to the number of member countries that will remain in the EU after the U.K.'s departure.

The leaders' stance puts them at odds with the man seen as Cameron's likely successor, Boris Johnson, who said Friday that he did not think it was necessary to rush Britain's exit from the EU.

"In voting to leave the EU it’s vital to stress that there’s no need for haste, and as the prime minister has just said nothing will change in the short term except work will begin on how to extricate this country from the supranational system. As the prime minister has said there is no need to invoke article 50," he said.

Article 50 is the law that allows countries to leave the EU, and a future British leader would have to invoke it in order to begin exit talks with his or her EU counterparts. Cameron said Friday that he would not be doing so, saying the timing should be left for his successor to decide.

The reactions of other world leaders to the British vote has been mixed.

German Chancellor Angela Merkel said that Germany would “take note of the British people's decision with regret. There is no doubt that this is a blow to Europe and to the European unification process.”

Others, however, were more upbeat, like far-right Dutch politician Geert Wilders congratulated the U.K. on the result, adding, “Now it's our turn.”

In addition, a spokesman for Russian President Vladimir Putin expressed hope that Britain's departure from the EU might smooth relations between Russia and the bloc.

"We have a pretty heavy burden of uneasy ties with Great Britain. We hope that in the new realities, an understanding of the need for good relations with our country will prevail," he said in a statement.