Speaking after Hungary's national elections, president of parliament's S&D group Hannes Swoboda said, "However big a majority Orbán has obtained, this does not absolve him from complying with EU law and respecting EU values".
96 per cent of the ballots were counted on Sunday night and an official projection gave Orbán's Fidesz party 133 of the 199 seats in parliament.
This means Fidesz will have a two-thirds majority, which is the required level for the party to push through constitutional change. However, this majority is only one seat over the threshold, and final results could still push Orbán's party back below the two-thirds level.
"Orbán has spent the last few years redrafting the constitution and passing laws that were frequently beyond what is acceptable in a European democracy", stressed Swoboda.
"Therefore, the European Union must continue to closely monitor whether Orbán's new government will respect European law. To this end, a monitoring group as proposed in the report adopted by the European parliament in July 2013 should be urgently established."
"Orbán has spent the last few years redrafting the constitution and passing laws that were frequently beyond what is acceptable in a European democracy" - Hannes Swoboda
In his previous four-year term, Orbán clashed repeatedly with the EU, especially on increased government control of the media, but many Hungarians regard the former anti-communist as a champion of their national interest.
In his time as prime minister, personal income tax and household power bills fell. Orbán's policies have also included a nationalisation of private pension funds, a levying of 'crisis taxes' on business and a relief scheme for mortgage holders for which the, mostly foreign-owned, banks were required to pay.
His party pledged more of the same if re-elected, and the business community expects him to press ahead with a plan to transfer part of the banking sector into Hungarian hands, and impose more burdens on foreign energy firms.
"The people of Hungary have renewed their confidence in Orbán and his government because he has always spoken the truth to them and because he implemented courageous reforms, which put the country's economy back on track" - Joseph Daul
While Orbán's policies have helped Hungary emerge from recession, some economists say he may have scared off the kind of investment Hungary needs for long-term growth.
Greeting his supporters on Sunday, Orbán called the result a "heaven-shaking" victory, as his party gained the support "greater than that of any European party".
"Hungary is the most unified nation in Europe… The voters have unequivocally put an end to all the disputes of the last four years", Orbán told his supporters.
He added that voters had said "no" to, what he termed, "hate" politics and also rejected leaving the European Union.
Further comment came from president of parliament's EPP group Joseph Daul, who congratulated Fidesz and party leader Orbán for their victory in the Hungarian elections.
"I warmly congratulate Viktor Orbán and Fidesz for [this] impressive electoral victory. The people of Hungary have renewed their confidence in Orbán and his government because he has always spoken the truth to them and because he implemented courageous reforms, which put the country's economy back on track."
"I am convinced… electoral victory by Fidesz will also be repeated in the European elections", said the French deputy.