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Votes cast by "baby boomers" in the 2015 general election outnumbered those cast by younger "millennial" voters by 4.2 million, research suggests.

Analysis of an election survey suggests 10.6 million voters aged 50-69 voted - about two-thirds of Britons that age.

But only 6.4 million - 46% - of Britons aged 18-24 voted, the study suggests.

The findings, based on the British Election Study, show a trend that began in the mid-1990s when the generational turnout gap began widening, continues.

However, part of the reason for the gap in number of votes is because there are more older voters.

Turnout as a proportion of voting age population Year 21-35 year olds 66-80 year olds 1964 71% 74% 2005 44% 70% 2015 47% 72%

Concern about apathy among younger voters has been a feature of the past few general elections, and organisations such as Bite the Ballot have been set up to try to get more young people on the electoral register.

But research from the Resolution Foundation suggests that the gap continues to be much wider than in elections of yesteryear.

'Deep-rooted problem'

It notes that there was just three percentage points difference in turnout between 66-to 80-year-olds and those aged 21-to-35 in 1964, when the study was first carried out.

That had widened to 26 percentage points in 2005 - slightly narrowing to 25 percentage points in 2015.

While they are not precise figures, the Resolution Foundation's findings are based on the BES, a survey which has taken place immediately after every general election since 1964.

The foundation measures turnout based on those who voted as a proportion of the voting age population - rather than those on the electoral register.

The report's authors point out that in 1992 - the last general election before turnout figures among younger voters began plummeting - 74% of 21-to-35-year-old voters surveyed said they cared "a great deal" which party won.

In 2015, that had dropped to 56%.

'Democratic engagement'

Over the same period, the figure rose from 78% to 81% for voters aged 66 to 80.

Laura Gardiner, senior research analyst at the Resolution Foundation, said: "This poor turnout among young people is a deep-rooted problem dating back to the mid-90s when young generation X-ers started turning away from the polling booth.

"This generational divide in turnout matters for our democracy but also has profound implications for policy if politicians feel they only need to target the votes of older generations to win power."

The government said there were 46.5 million people on the electoral register "the highest figure at any point in its history" but it recognised that electoral registration was a "social justice issue".

Minister for the Constitution Chris Skidmore added: "We will continue to engage more and more young people in democracy through support to energetic campaigns, and this autumn I will set out in detail the government's vision for driving up democratic engagement."