It says the rising appeal of populist parties could see some winning elections and mainstream parties forced into previously unthinkable alliances. 

Europe's "crisis of democracy" is a gap between elites and voters, EIU says.

There is "a gaping hole at the heart of European politics where big ideas should be", it adds.

Low turnouts at the polls and sharp falls in the membership of traditional parties are key factors in the phenomenon.

'Highly destabilising'

The United Kingdom - going to the polls in May - is "on the cusp of a potentially prolonged period of political instability", according to the Economist researchers. 

They say there is a much higher than usual chance that the election will produce an unstable government - predicting that the populist UK Independence Party (UKIP) will take votes from both the Conservatives and Labour.

The fragmentation of voters' preferences combined with Britain's first-past-the-post electoral system will, the EIU says, make it increasingly difficult to form the kind of single-party governments with a parliamentary majority that have been the norm. 

But the most immediate political challenge - and test of how far the growing populism translates into success at the polls - is in Greece. A snap general election takes place there on 25 January, triggered by parliament's failure to choose a new president in December.

Opinion polls suggest that the far left, populist Syriza could emerge as the strongest party. If it did and was able to form a government, the EIU says this would send shock waves through the European Union and act as a catalyst for political upheaval elsewhere.

"The election of a Syriza government would be highly destabilising, both domestically and regionally. It would almost certainly trigger a crisis in the relationship between Greece and its international creditors, as debt write-offs form one of the core planks of its policy platform," the EIU says.

"With similar anti-establishment parties gaining ground rapidly in a number of other countries scheduled to hold elections in 2015, the spill-over effects from a further period of Greek turmoil could be significant."

'Immigration and austerity'

Other examples of European elections with potential for unpredictable results cited by EIU include polls in Denmark, Finland, Spain, France, Sweden, Germany and Ireland.

"There is a common denominator in these countries: the rise of populist parties," the EIU says,

"Anti-establishment sentiment has surged across the eurozone (and the larger EU) and the risk of political disruption and potential crises is high."

Its analysis is that populist parties and movements - of the left, the right and the indeterminate - are moving into the space that has opened up between the old political parties and their traditional social base.

Opposition to governance from Brussels, immigration and austerity are key themes and rallying cries for many of these parties.

'Upsurge of protests'

Meanwhile, alongside the rise to prominence of populist movements, there has been an upsurge of popular protest in many parts of the world in recent years.

The EIU estimates that significant protest movements surfaced in more than 90 countries during the past five years - in the main, it says, led by young, educated, middle class individuals who resent their political leaders and who prefer Twitter and other social networks to the traditional political soap box.

"An upsurge of popular protest has swept through Europe, the Middle East, North Africa and Latin America in recent years. Other regions such as Asia and North America have been less susceptible, although have not escaped entirely," the EIU says.

"The mainsprings of the protests have been different - some have been responses to economic distress, others are revolts against dictatorship; some are expressions of a popular desire to have their voices heard by political elites, others express the aspirations of new middle classes in fast-growing emerging markets."

BBC Democracy Day

• Democracy Day takes place on Tuesday 20 January, across BBC radio, TV and online
• A look at democracy past and present, encouraging debate on its role and future
• 2015 marks the 750th anniversary of the first parliament of elected representatives at Westminster
• It also sees the 800th anniversary of Magna Carta - a touchstone for democracy worldwide
• Go to the BBC News website's Democracy Day page, for analysis, backgrounders and explainers on the debate

All these developments pose the question of whether they do actually represent a threat to democracy or are proof that it is alive and well.

If we take the Arab Spring as a springboard for democracy in a region where it was notable for its absence, the result so far of course has been decidedly limited democratic change and a great deal of upheaval.

And the impact of the pro-democracy movement in Hong Kong, for example, is yet to become clear - protesters there dismissing Beijing's proposals for the way in which Hong Kong will be able to choose its own leader as "fake democracy". 

And in the world's most populous nation itself, Western-style democracy is nowhere on the horizon - though whether that position can and will be sustained indefinitely in the face of democratic trends elsewhere in Asia and the expansion of China's middle class is another question. 

For now, the unpredictable fate of the "old" democracies will undoubtedly be watched closely by governments and activists of all political hues around the world.