David Cameron has told other EU leaders that it "cannot be business as usual" in Brussels following the success of eurosceptic parties in the European elections.
The Prime Minister's spokesman said Mr Cameron had spoken to leaders including Germany's Angela Merkel and the Irish Taoiseach, Enda Kenny.
He will also hold talks tomorrow at a dinner in Brussels tomorrow evening.
"The PM has been making the point in these conversations that leaders should seize the opportunity of tomorrow's dinner to heed the views expressed at the ballot box that the EU needs to change and to show it cannot be business as usual," the spokesman added.
In the Netherlands, the anti-Islam, Eurosceptic Dutch Freedom Party of Geert Wilders' - which plans to forge an alliance with Le Pen - finished joint second in terms of European Parliament seats behind a pro-European centrist opposition party.
In Italy, Prime Minister Matteo Renzi's centre-left Democratic Party was holding off a strong challenge from the anti-establishment 5-Star Movement of former comic Beppe Grillo, according to a first exit poll.
In Germany, Chancellor Angela Merkel's Christian Democrats were set to secure 36 percent of the vote, down from a 23-year-high of 41.5 percent in last year's federal election but still a clear victory. The centre-left Social Democrats, her coalition partners, were forecast to take 27.5 percent.
The anti-immigration Danish People's Party became the country's biggest party in Europe, echoing successes for the political right across much of the bloc in elections to the European Parliament, preliminary results show.
While the Nordic region remains liberal and open, right wing parties' portrayal of welfare benefits under threat from immigration have struck a chord with some voters, also helping the Sweden Democrats to win EU seats for the first time.
Finland's right wing The Finns Party, won a second EU seat, but fell short of pre-vote forecasts, preliminary results showed.
Marine Le Pen's far-right National Front have stunned France's political elite by taking first place in European Parliament elections, with President Francois Hollande's Socialists beaten into a poor third, provisional results showed.
It was the first time the anti-immigrant, anti-EU party had won a nationwide election in its four-decade history.
If the results are confirmed, it could secure as many as 25 seats in the new European Parliament, more than eight times the three it won in 2009.
"The people have spoken loud and clear," a triumphant Le Pen told cheering supporters at party headquarters in a northwestern suburb of Paris.
National Front leader Marine Le Pen called for the dissolution of the French National Assembly after exit polls showed her anti-immigrant, Eurosceptic party winning European Parliament elections in France.
"What else can the president do after such a rejection?" Ms Le Pen said.
"It is unacceptable that the assembly should be so unrepresentative of the French people."