More than 9 million people who are eligible to vote in the UK are not correctly registered and are at risk of not being able to have their say in a potential snap election, according to research.

The finding sparked renewed calls for Britain to follow Canada and Finland, among other countries, who automatically register voters. One other potential model would enable people to opt in when they engage with government bodies such as the DVLA, NHS and welfare agencies.

Research by the Electoral Commission analysed electoral registers and found that 17% of eligible voters in Great Britain, as many as 9.4 million people, were either missing from the electoral register or not registered at their current address, with major errors affecting up to 5.6 million people.

It highlighted stark differences in registration levels between younger people, renters, low-income and black and ethnic minority people, compared with older white people who own their homes.

The study also showed that the number of people not correctly registered had risen from 16% of eligible voters in 2015, representing as many as 8.3 million people.

“These figures should sound the alarm for anyone who cares about democracy,” said Dr Jess Garland, from the Electoral Reform Society (ERS). “That more than 9 million people are not correctly registered is a major barrier to political equality and democratic engagement. It means any snap election will almost certainly be on the basis of an incomplete franchise, with millions missing.

“You shouldn’t have to opt in to your right to vote. As the Electoral Commission note, we need to move towards automatic registration now, starting with being able to check you are registered online, and being able to register whenever you engage with government bodies or services. We know this works from other countries.”

The ERS has also called for same-day registration systems to be trialled, allowing people to sign up on election day, as well as an online service to find out if people are registered.

Georgie Laming, from the housing campaign group Generation Rent, said renters in the UK frequently moved home, often due to rising rents and lack of security, and that making registration easier would help the 11 million private renters have their voices heard.

“There are some simple ways that registering to vote can made easier for renters, by integrating registration with services lots of renters use like paying their council tax locally,” she said.

In 2014, the government introduced individual voter registration to reduce the possibility of electoral fraud. This prevented universities from registering students en bloc.

Earlier this year, voter ID was made compulsory in 10 voting districts despite voter impersonation making up just 3% of all alleged electoral offences at the previous local election.

Labour announced in April it would consider introducing automatic voter registration to increase election turnout.

Bridget Phillipson MP, who represents the Speaker’s committee on the Electoral Commission, said this month it was impossible to determine the number of people entirely missing from the registers because there was not a “consistent, unique identifier for individual electors on the registers”.

More than 100,000 people applied to register to vote in two days this month, with young people making up the bulk of the surge. However, it followed an almost 1% overall drop in those registered to vote between 2017-18.

• This article was amended on 27 September 2019 to make clear that the research was conducted by the Electoral Commission, rather than the Electoral Reform Society.