The UK’s electoral system is an “open invitation to malicious influencers” because of its fixation with marginal constituencies, proportional representation campaigners warn.

In the wake of the Cambridge Analytica scandal, campaign group Make Votes Matter warns that Britain’s First Past the Post voting system is highly vulnerable to online influence, and even to interference by foreign powers.

Analysis by the group reveals that tiny changes in voting behaviour in key geographical areas could have dramatically changed the results of the 2017 General Election. This creates the perfect environment for illicit social media campaigning to swing election outcomes.

The cross-party campaign’s analysis points out that the Conservative Party would have won an overall majority if just 533 people had have voted differently in the right places, whilst Jeremy Corbyn would have won a majority if just 0.16% of voters had opted for his party over the Conservatives (51,072). 31 constituencies were won with a majority of fewer than 500 votes.

Political parties have long tailored their messaging and resources to target marginal constituencies. On average 37.5% more was spent on election campaigns in marginal seats than in safe seats in the last General Election, according to the Electoral Commission.

However, allegations about Cambridge Analytica’s Facebook data mining, as well as the alleged influence of Russian social media accounts on the 2016 US Presidential Election, suggest the British voting system is highly vulnerable to nefarious influence.

Owen Winter from Make Votes Matter said: “For years political parties have known that because of our First Past The Post system, the keys to 10 Downing Street lie in the hands of a tiny minority of swing voters in marginal seats.

“Now we face a fresh threat to democracy. Shadowy groups are exploiting technology – data harvesting, targeted ads, bots and fake news – to ruthlessly target and influence those voters without oversight or accountability.

“With a proportional voting system which makes every vote count, it would take millions of voters to alter the final result. But in our laughably outdated voting system, targeting and influencing a few voters in the right places can decide a government.”

The group argues that a system of Proportional Representation would help protect the UK from external interference by ensuring that a tiny group of swing voters do not hold the keys to power.