First-time voters could unseat their MP in 56 marginal seats across the country, according to an exclusive analysis of the 1.2m new electors who have come of age in England and Wales since the 2017 general election.

Thirty of these seats are held by the Conservative party, 20 are held by Labour, four by the Liberal Democrats (almost a fifth of their seats) and two by Plaid Cymru.

Of the 20 seats where the number of new voters exceeds the incumbent MP’s majority by the biggest margin, nine belong to the Conservatives, nine to Labour, and two to Plaid Cymru, according to the statistical analysis by the Intergenerational Foundation (IF).

“Under Britain’s first-past-the-post electoral system some votes really are much more important than others; in any given election only a small minority of the most marginal constituencies actually change hands,” said Angus Hanton, the co-founder of the IF. “Winning British elections is about winning marginals.”

Last Friday, when the BBC hosted a leaders’ debate, 206,011 young people under the age of 34 registered to vote – 102,768 of whom were under 25, according to the most recent data. At the time of writing, more than 670,000 people under 34 registered to vote in the previous seven days.

With the registration deadline only two days away, the Electoral Commission has made a final plea, telling people the process can be done while “waiting for the kettle to boil”.

“It only takes five minutes to register online,” said Craig Westwood, the director of communication, policy and research at the Electoral Commission. “If you want to make sure your voice is heard, and you’re not already registered, go online and register now.”

Online registration closes on midnight on Tuesday. Applications for post, postal proxy applications and for changes to existing postal or proxy votes need to be made by 5pm the same day.

There are still up to 9.4 million people missing from the electoral roll. Without a huge final push, warned Willie Sullivan, the senior director (campaigns) at the Electoral Reform Society, many young people will be left out of the decision.

“These new figures show just how tight this election could be and the importance of voters, particularly young people and other hard-to-reach groups, getting registered to vote,” he said.

Millennials – defined as people born between 1981 and 1996 – are significantly less likely to participate in electoral politics than older generations. Until recently, the UK had one of the largest gaps between the electoral participation of older and younger people of any developed country.

All the seats identified in IF’s research are highly marginal; the size of the incumbent’s majority varies from only 20 in Labour-held Kensington to 2,690 in Conservative-controlled Corby. The 10 constituencies where new voters could make the biggest difference are Kensington, Dudley North, Newcastle-under-Lyme, Southampton (Itchen), Richmond Park, Crewe and Nantwich, Arfon, Ceredigion, and Canterbury.

The statistical modelling also found that there are 28 marginal seats where the incumbent MP’s majority is less than 10% of the number of potential voters aged between 18 and 34, with the Kensington constituency having the tightest margin.

The latest polling data suggests that the very significant differences in how voters of different ages cast their ballots at the 2017 general election are likely to be maintained; YouGov, polling at the end of October, gave Labour a sizeable lead among potential voters aged 18 to 24.

Although the specific situation will vary considerably from seat to seat, Hanton said, this suggests a higher turnout among new voters in any of these marginals would likely benefit Labour significantly more than it would the Conservatives.

“This data is not meant to suggest either that first-time voters can dramatically shift the outcome of the whole election, or that those who live in safe constituencies shouldn’t bother to vote,” he said. “It is merely meant to illustrate the point … that young people hold more power than probably either they, or for that matter many politicians, have realised, and like all voters their influence is magnified if they happen to live in a marginal seat,” he said.

Charlie McCurdy, a researcher at the Resolution Foundation, said: “The average age of local residents is increasingly determining who represents them in Westminster. The oldest and youngest parliamentary constituencies have become safer for their respective parties in recent elections. The risk is that this causes the main parties to double down on their core bases – turning Labour into the millennials party and the Conservatives into the pensioner party.”