The leaders of the Green party and Ukip have forged an unlikely political union in order to call for a fairer electoral system, after their parties gained 16.4% of the vote in the general election but emerged with just 0.31% of the seats in the House of Commons.

Natalie Bennett, the Green party leader, appeared alongside the Ukip leader, Nigel Farage, and the party’s only MP, Douglas Carswell, on Monday to call for an electoral overhaul, in an event organised by the Electoral Reform Society and Unlock Democracy.

Facebook Twitter Pinterest Ukip leader Nigel Farage on Monday. Photograph: EPA

They were joined by representatives from the Liberal Democrats, the Scottish National party, and Plaid Cymru, all of whom signed a 477,000-signature petition at Westminster’s Old Palace Yard, before handing it in to 10 Downing Street.

A Green party spokesperson said the Greens and Ukip were the most prominent parties in the campaign because they had been the most unfairly treated by the system.

Despite receiving 12.6% of the vote in the general election – making them the third-biggest party in terms of votes – Ukip won only one seat in parliament under the first-past-the-post system. The Greens received 3.8% of the vote and only one seat, while the Liberal Democrats won 7.9% of the vote and only eight seats.

The SNP did comparatively well out of the current electoral system, having won 4.7% of the vote but 56 MPs. Plaid Cymru, which received only 0.6% of the vote, has three seats.

If the election had been conducted with a system of proportional representation, Ukip would have been the third-biggest party, after the Conservatives and Labour, with about 82 seats. The Liberal Democrats would have won about 51 seats and the Greens about 24 seats. The SNP would have picked up only about 31 seats.

Farage condemned the fact that 5m votes (those of Ukip and the Greens combined) and the views of 5 million people, were represented by only two MPs. “It cannot go on like this. It is important to be here with the Electoral Reform Society supporting its campaign to make votes match seats.”

We haven’t seen significant reform at Westminster since women got the vote Natalie Bennett

Bennett said the election had demonstrated the need to change the constitution. “We haven’t seen significant reform at Westminster since women got the vote, and that was 1918 – we shouldn’t get to the centenary of that without getting a fair voting system,” she said.

“The SNP has done well under first past the post this time, but we have always supported proportional representation and will continue to do so,” said the SNP’s Westminster leader, Angus Robertson MP. “From an unrepresentative voting system to the unelected House of Lords, SNP MPs will be a strong voice in the coming years for the change we need to see.”

After a week in which competing factions within Ukip were embroiled in a public row over Farage’s future as leader, he seemed keen to demonstrate a united front with Carswell. “I’m also delighted to be joining with Douglas Carswell, who has campaigned for political reform for years and is leading the Ukip charge for this in the House of Commons,” he said.

Carswell’s interest in electoral change dates to before he defected from the Conservative party in August 2014. In 2010, while a Tory backbencher, Carswell teamed with the Green party’s only MP, Caroline Lucas, to campaign for proportional representation to be included as an option on the ballot paper during the referendum on the possible change to the alternative vote electoral system.