British politics is no longer as simple as left against right, according to research highlighted by senior Conservative and Labour strategists.

Traditionally loyal voters have never been more prepared to cross party lines, said Andrew Cooper, David Cameron’s former director of strategy, and Spencer Livermore, a Labour adviser on four election campaigns, who described how shifting attitudes have created new divisions in the UK.

“Open versus closed is becoming more and more significant,” said Lord Cooper, launching a report with polling that suggests it is easier to predict voting habits by how internationalist people are, whether they live in diverse communities, their feelings towards minorities and their age.

“It means that lots of people who once were good bets to be Conservative now turn out to be Labour. And people that pollsters might have predicted would be Labour are now Tories,” he said.

An analysis carried out by Cooper using the polling company Populus on behalf of the thinktank Global Future laid bare a stark divide between people aged 18 to 44 and those over 45, with a huge gap in attitudes to internationalism, multiculturalism and immigration.

In comments that suggest the trend could be particularly worrying for Theresa May’s Conservatives, Cooper argued that there was no good reason to believe the “generation that has come of age in the last 25 years is going to change its worldview as it grows older”.

Unlike previous shifts, where people have become more conservative in their 50s and 60s, he said the “open worldview is baked in, it is not a function of life stage”.

Lord Livermore described the research as “by far the most compelling and rigorous explanation” for the 2017 general election result, when the age at which people shifted from Labour to Conservative rose from 34 to 47.

“He shows where old alliances are breaking down and new electoral coalitions are beginning to be formed. He outlines how the real division in British politics now is not between left and right, so much as being open to the world or closed,” he said.

“And he explains why there was such a significant swing to Labour, and away from the Conservatives, among voters under the age of 45. The axis of British politics is rotating and, as it does [so], it presents Labour with a huge opportunity.”

Livermore argued that the route to government was through “progressive, inclusive politics”.

Global Future supplemented the polling by a demographic model from Populus based on census data. This looked at factors such as income, occupation, housing tenure and health as well as ethnicity, immigration and whether people lived in urban or rural areas.