The SNP, Greens and Welsh nationalists have united to call for an electoral pact with Labour and the Liberal Democrats to agree sweeping changes to the voting system for Westminster elections.

In a letter published in The Independent on Sunday, the leaders of the three left-of-centre parties call for an agreement on electoral reform to be included in each of their manifestos in 2020.

It comes after The IoS revealed last week that Jeremy Corbyn and the Liberal Democrat leader Tim Farron had opened talks on electoral reform to agree changes before the general election as part of a wider progressive electoral alliance against the Conservatives.

Mr Farron’s aides are talking to a Labour MP – a close ally of Mr Corbyn – who is acting as a conduit between the two leaders.

But a senior Liberal Democrat source said the SNP – the third biggest party in Parliament with 54 MPs – could also be involved in the talks. The source said if the negotiations are successful up to five left-of-centre parties could stand on an agreed platform of voting reform at the 2020 election – giving them a mandate to scrap Westminster’s first past the post system without a referendum if they can secure a majority in the Commons.

In the letter, the SNP leader in Westminster Angus Robertson, Hywel Williams MP, the parliamentary leader of Plaid Cymru, and the Green MP Caroline Lucas welcome the talks between Labour and the Liberal Democrats.

They write: “By working together we believe it’s possible to transform British politics – and that a fairer voting system will help deliver a fairer Britain.

(Rex)

(Getty)

From top: the SNP leader in Westminster, Angus Robertson, the Green MP Caroline Lucas, and the leader of Plaid Cymru, Hywel Williams MP welcome the talks between Labour and the Liberal Democrats (Rex,Getty)

Last year’s general election delivered an extremely unrepresentative House of Commons, with huge numbers of voters effectively not having a say. A third of people who voted backed neither Labour nor Conservative candidates, yet their votes translated into just 13 per cent of the MPs.

“The Conservatives achieved a Commons majority on the basis of just 37 per cent of the votes cast. This is plainly unjust and undemocratic.”

The three Westminster leaders claim that reforms introduced by the Government to change constituency boundaries and the process for registering to vote will make the situation “even worse”.

They add: “In this situation, it is vital for democrats to work together and make every person’s vote count, despite our differences in many other areas of policy.

“We would like to see Labour, the Liberal Democrats and the parties we represent joining together at the next general election in a joint manifesto pledge to introduce proportional representation for the House of Commons. This would give every voter a say in future elections and provide a mandate for the early introduction of legislation.”

The three MPs pledge to work with other party leaders further “about how this pledge can be achieved”.

The Liberal Democrats are understood to want a respected elder statesman in the Labour Party to take on a formal role as a go-between between the parties.

The party is still scarred by the humiliating 2011 referendum on introducing the “Alternative Vote” system for elections to the Commons. The final result as a one-sided 68 per cent to 32 per cent victory for the “no to AV” campaign, ending any hopes of changing the voting system through a referendum.

Mr Corbyn has admitted that he is “open” to an electoral pact with other parties to force through electoral reform.

Asked if he would be willing to talk to the Liberal Democrats and other parties about agreeing changes to the voting system, the Labour leader said he “could be”. But pressed whether he was open to it, he replied: “Obviously.”

Mr Corbyn said he was willing to consider a form of proportional representation as long as it maintained the link between an MP and their constituency. He added: “If parties are getting less than 40 per cent of the vote, that seems to be the trend from the last four elections, we have to recognise that.”

However, Mr Corbyn will face a backlash from many of his own MPs to any proposal to work with the Lib Dems or change the voting system.