Eurosceptic parties are set to win a third of the seats or more in May's European Parliament elections, warned a study by the European Council on Foreign Relations (ECFR).
The study said the elections could "see a group of nationalist anti-European political parties that advocate a return to a 'Europe of the nations' win a controlling share of seats in the European Parliament".
The pan-European think-thank defined "anti-Europeans" as members of political parties from the far-left to the far-right, including right-wing Eurosceptics.
The rise of these parties could "undermine and cause irreparable damage to areas such as trade, security, and defence," it said.
The ECFR findings were based on interviews with political parties, policymakers and policy experts, as well as by analysing opinion polls in the 27 member countries. The think-tank conducted the calculations assuming that the UK would not participate in the May elections.
The study warned that if these parties do gain more seats in the European Parliament, their ideas could shape European policy in the medium-term, and "paralyse decision-making" in the long term.
"If nationalist parties marshal the clearest, loudest arguments and significant numbers of their supporters turn out to vote, the views of Europe's silent majority will be drowned out in the new parliament," it said.
According to the report, the far-right is set to rise from 23% to 28% and could cross the 30% threshold "if their popularity continues to grow," which would "signify a qualitative change in the EU".
The rise of the far-right in the elections could even see the establishment of a new political group, which could be "the second-largest political family in the EP," the think-tank said.
The ECFR's research found that anti-EU parties mainly pose a threat to the bloc in the areas of rule of law by blocking Article 7, foreign policy, security, trade by obstructing free-trade agreements, and migration.
If progressives wish to stop anti-EU parties from gaining more seats they will have to be more open to compromise with one another in order to "defend the European project" and try to "preserve cohesion of their political groups to avoid losing their distinct identities," the report said.
ECFR's advice to countering the anti-European movement include exposing the real-world costs of the anti-EU agenda and inspiring voters with new issues that could convince them to vote.