Nearly 14 million voters are living in constituencies that have been held by the same political party since at least the second world war, according to an electoral reform campaign group, which said this illustrated a “broken” voting system.

Highlighting what it says is the inflexibility of the first-past-the-post (FPTP) system, where single MPs are picked per constituency on a non-proportional basis, the Electoral Reform Society (ERS) said the average seat had not changed hands for 42 years.

If equivalent predecessor seats are taken into account, some constituencies have not seen a change in party for their MP since the era of the Whigs in the 19th century. Eleven Labour seats and 54 held by the Conservatives have not changed hands in more than a century.

At the 2017 election, only 70 seats, or 11% of the total, went to a different party, with this figure in gradual decline. According to a YouGov seat-by-seat projection for the 12 December, 58 seats are due to change hands.

This comes despite current voter volatility unprecedented in recent decades. In October, findings from the ongoing British Election Study found that almost half of voters have switched sides since 2010 amid political upheavals such as Brexit.

A report this year by the Constitution Society thinktank argued that FPTP does not even reliably carry out the role argued by its supporters of keeping out extreme elements and encouraging moderate, consensual politics.

The report said recent elections which saw support for the Conservatives and Labour concentrated into defined geographical areas meant there was less direct competition between them, pushing both parties away from the centre ground, aided by the increasing influence of activist members.

The ERS study found that 98 Labour seats have been in the party’s hands since the war, 37% of their 2017 total, with 94 Conservative seats in the same situation, 30% of the constituencies they won at the last election, affecting 13.7m potential voters overall.

In total, 10% of current seats, 65, had been held by the same party since 1918 or earlier. The Devon seat of Hugo Swire and its prior equivalent last changed hands in 1835, when the Conservatives took it from the Whigs.

The seats occupied by Theresa May and Chris Grayling, Maidenhead and Epsom and Ewell, have been held by the Conservatives since 1874.

Jess Garland, head of policy for the ERS, said: “We’ve heard often that politics is volatile and anything could happen in the coming election, but even so, hundreds of seats across the country haven’t changed party hands for decades.

“Huge parts of this country are effectively competition-free zones, with ‘safe’ seats leaving voters demoralised and ignored time and again.

“When there are seats in this country that have not changed party since before Queen Victoria was on the throne, it’s clear that we need a change. People need to know their vote will count, whichever party they give it to, and wherever they are in the country. Unfortunately, under first past the post, some votes count much more than others.”