Brussels - The crushing re-election victory of Hungary's Viktor Orban may have delighted populists around Europe, but the anti-immigration hardliner's triumph looks set to prolong the fractious relationship between Brussels and Budapest.

Congratulations for Orban from EU Council President Donald Tusk, European Commission chief Jean-Claude Juncker and the leading political group in the European Parliament were tempered by warnings that Hungary must live up to the bloc's values.

The EU has clashed repeatedly with Orban's government during its past two terms over a number of issues including Hungary's crackdown on foreign-funded universities and NGOs, its hardline stance on migrants and alleged misuse of funding from the bloc.

Juncker's spokesman Margaritis Schinas said the former Luxembourg PM would congratulate Orban in a letter and phone call, but also struck a warning note.

"The European Union is a union of democracies and values. President Juncker and the commission believe that the defence of these values and principles is a common duty for all member states, without exception," Schinas said.

In a terse congratulatory note, Tusk told Orban he was counting on him to "play a constructive role in maintaining our unity in the EU".

The EU has launched a barrage of legal action against Orban's government, taking it to the bloc's top court over a crackdown on education and foreign-backed civil society groups that critics say targets US billionaire George Soros.

The European Commission has also brought a case against Hungary, Poland and the Czech Republic over their refusal to accept quotas for asylum seekers.

But Orban has remained defiant in the face of both criticism and legal action from Brussels and the bloc faces a challenge in trying to either cajole or compel him to cooperate.

- 'Talk straight to him' -

Orban's Fidesz party is part of the European People's Party (EPP), the dominant force in the European Parliament which also includes German Chancellor Angela Merkel's CDU party, and some critics say this has given him political cover in Brussels.

On Monday Manfred Weber, the head of the EPP bloc in the parliament, congratulated Orban on Twitter, saying he was looking forward to "continuing working with you towards common solutions to our European challenges".

Left-leaning groups in parliament condemned the EPP's support for Orban, with German MEP Sylvia-Yvonne Kaufmann, of the Socialists and Democrats, saying they needed to "talk straight to him, instead of flattering him with visits and invitations".

It has been suggested the EU could launch so-called Article 7 sanctions over Hungary's alleged breaches of the bloc's values, which could strip it of voting rights, but such a move would almost certainly be vetoed by Budapest's ally Poland.

Daniel Mikecz of the Republikon Institute, a Hungarian thinktank, said that on the European stage, if not domestically, Fidesz could be open to persuasion by the EPP.

"Fidesz value their membership of the EPP and their good relations with the German conservatives -- those two actors could be a moderating influence," Mikecz said.

Similarly, Agoston Mraz of the Nezopont Group thinktank said much would hinge on a meeting Orban has proposed with Merkel and the leaders of Poland, Slovakia and the Czech Republic -- which make up the Visegrad Four group with Hungary.

In an interview with AFP, Weber insisted that the EPP's backing for Orban was not unconditional, stressing that Fidesz must respect the "fundamental values" of the EU.

"On one hand there is the election. We must respect the outcome of the decision made by the voters. On the other one, we have a clear request and clear expectations to what Viktor Orban must deliver to be part of the EPP," Weber told AFP.

But Fidesz has already pledged to forge ahead with a new "Stop Soros" package of legislation, setting the stage for fresh clashes with the EU.

- Massive victory -

Orban's massive victory -- with around 49 percent of the vote and potentially gaining a two-thirds majority in parliament -- comes after eurosceptic and anti-EU parties made big inroads in elections in Austria and Italy.

Praise for Orban has flooded in from allies such as Poland, far-right leaders like Marine Le Pen of the French National Front and staunch anti-EU campaigners like Nigel Farage, former head of Britain's UKIP.

Joachim Fritz-Vannahme, an analyst with the Bertelsmann Stiftung thinktank, said Orban's win could be an unpleasant foretaste for Brussels of next year's elections to the European Parliament.

"We already have almost a third of MEPs who are anti-Europe or eurosceptic, from UKIP to Marine Le Pen," he told AFP.

"If we take the results in Italy, Hungary and Austria it's clear there is a very strong anti-European current which is going to dominate the character and temperament of the next European Parliament."