An unprecedented period of volatility in party loyalties, in which almost half of voters have switched sides since 2010, and the fluidity of Brexit will make the next general election hugely difficult to call, according to a major study.

The findings from the British Election Study (BES), an ongoing survey running since the 1964 election, augmented by ongoing polling from a 30,000-person online panel, will make uneasy reading for party leaders plotting their strategy.

It shows that following decades of relative stability, voter decisions have been rocked repeatedly by a series of recent “electoral shocks” – immigration and the rise of Ukip; the 2008 crash; the 2010-15 coalition; the Scottish referendum and Brexit.

Amid the repeated shake-ups, traditional party loyalties seem to have eroded. The study found that in the 2015 election, 43% of people voted for a different party than in 2010, bringing a record high share of the vote for parties other than the main three.

Two years later, 33% of people switched for the 2017 poll, and the Conservatives and Labour won more than 82% of the votes. In this election there was also the biggest movement in voters between the two main parties detected in BES data.

Across the three elections of 2010, 2015 and 2017, 49% of people changed votes at least once.

The study concludes that only two recorded elections have had higher aggregate volatility than 2015 and 2017, both of which took place in deeply unusual circumstances.

The first was in 1918, after the first world war and following the expansion of the voting franchise. The other was in 1931, when a post-depression national government won a landslide.

The authors of the research, which is to be collated into a book published this year, say this context, plus the fact that Brexit will still be a live issue in the coming months, makes the likely imminent next election hugely difficult to call.

“Given the UK’s recent history of vote switching and the unpredictability of the current climate, it would be unwise for any political party or commentator to presume how voters will behave in a general election, particularly in the middle of an electoral shock,” said Edward Fieldhouse, a politics professor at Manchester University. “But we do expect to see big shifts defined largely by Brexit.”

The idea of electoral shocks seeks to explain the extent of vote switching, which the researchers say goes beyond longer-term factors such as a gradual weakening in party loyalties, especially among new cohorts of younger voters.

The study found that the 2017 election had the largest recorded amount of voter movement between the two big parties. One in eight Conservative voters from 2015 backed Labour, with one in 11 former Labour voters moving the other way.

This totalled 11% of the electorate switching between the two parties. In contrast, during Tony Blair’s landslide election win in 1997, the figure was 9%.

The research suggests that the cumulative effect could be more volatility, as new electoral shocks shift already unattached voters. With the likelihood of a new departure extension before the next election, Brexit seems set to be a key factor – and one that is hard to predict.

“We don’t know what the Brexit situation will be on election day,” said Jane Green, professor of politics at Oxford University. “We don’t know who will get the blame for the current political deadlock, or who will benefit.

“But we do expect there to be clear winners and losers because voters are now more changeable in response to such shock events. A key driver of vote choice will be how competent each party is perceived to be on Brexit.”