Britain’s smaller parties have been urged to make reforming the UK’s “undemocratic” voting system a red line for supporting a government in a hung parliament.

The Electoral Reform Society (ERS) said the Liberal Democrats, Greens, SNP, Brexit Party, and Plaid Cymru should demand proportional representation if they hold the balance of power.

Tactical voting has been a major issue in this year’s election because the UK’s existing voting system, first past the post, penalises voters who support smaller parties.

“We don’t yet know the outcome of this election – but we already know that the results will be built on an undemocratic voting system,” said Darren Hughes, chief executive of the ERS. “Winner-takes-all results at Westminster have failed voters across the country for too long.

“With millions forced into tactical voting, hundreds of safe seats and electoral pacts denying millions more any real choice, it’s time to say enough is enough.

“Trust in politics is at rock bottom, and voters desperately need to be heard. Let’s make this the last election under the unjust electoral system. We urge all party leaders to put the country first, and commit to making real reform at Westminster a priority.”

All the parties contacted by the ERS support some form of proportional representation in their manifestos, though they have not said it would be a deal breaker for them.

In 2010, the Liberal Democrats secured a referendum on changing the UK’s electoral system to Alternative Vote (AV).

AV, which is not proportional but is designed to reduce the need for tactical voting, was rejected – in an episode that apparently killed off further opportunities for electoral reform for a decade.

In July, all of the parties in question signed a “Good Systems Agreement”, a cross-party consensus setting out the principles they believe a new proportional voting system should deliver.

They also endorsed a call for a citizen’s assembly to select the right system for the UK. The agreement, brokered by the campaign group Make Votes Matter, was also signed by a number of Labour, Conservative and independent MPs.

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Molly Scott Cato, a Green MEP who is standing for the Commons in Stroud, told The Independent: “I have never known such dissatisfaction with the ‘two main parties’. The two-party system stifles creativity and change and feeds the hyper-partisanship that is undermining our political culture.