The data shows that populism has been consistently on the rise since at least 1998. Two decades ago, populist parties were largely a marginal force, accounting for just 7% of votes across the continent; in the most recent national elections, one in four votes cast was for a populist party.

Populists tend to frame politics as a battle between the virtuous ‘ordinary’ masses and a nefarious or corrupt elite – and insist that the general will of the people must always triumph. The Guardian is adopting the classic definition of populism proposed by political scientist Cas Mudde. Populism, he says, is often combined with a ‘host’ ideology, which can either be on the left or right.

It reveals the different fortunes of rightwing populists such Hungary’s Viktor Orbán and Italy’s Matteo Salvini, who have had the most success in recent years, and leftwing populist parties, which rapidly expanded in the aftermath of the financial crisis but failed to secure a seat in government anywhere other than Greece.

Populism in Europe goes back several decades: the far-right Freedom party of Austria was founded in 1956 by a former Nazi and first won more than 20% of the vote in 1994. It is now part of the country’s ruling coalition.

Populist parties enjoyed success in Norway, Switzerland and Italy in the 1990s. But it was not until the turn of the century that populist ideas, legislators and challengers started to proliferate, from the Netherlands to France, Hungary to Poland.

Since then, anti-establishment populism has snowballed, particularly after the 2008 financial crash and the 2015 refugee crisis in Europe. The anti-austerity Syriza took 27% of the vote then 36% in successive Greek elections; Ukip propelled Britain to its Brexit vote and Marine Le Pen became the second member of her family to reach a presidential run-off in France, winning 33% of the vote.

Claudia Alvares, an associate professor at Lusofona University in Lisbon, who was not involved in the Guardian research project, said: “The success of such politicians has very much to do with their capacity to convince their audiences that they do not belong to the traditional political system. As such, they are on a par with the people to the extent that neither they nor the people belong to the ‘corrupt’ elites.” social media had a role to play in the rise of populism, its algorithmic model rewarding and promoting adversarial messages. “The anger that populist politicians manage to channel is fuelled by social media posts, because social media are very permeable to the easy spread of emotion. The end result is a rise in the polarisation of political and journalistic discourse.”