The Liberal Democrats, Plaid Cymru and Green party are set to unveil a general election pact in which the parties would stand aside in certain seats to boost each others’ chances.

An announcement expected next week on the pact, which is a successor to the so-called progressive alliance discussed before the 2017 general election that achieved little mainly due to local parties’ inability to agree who should stand down where.

The 2019 incarnation does not include Labour and thus would have a necessarily limited scope, as Plaid Cymru has just four MPs and the Greens one.

However, this variant has already had success, with Plaid and the Greens not standing candidates in August’s Brecon and Radnorshire byelection, allowing the Liberal Democrats a clearer run, helping them take the seat from the Conservatives.

The three-party process is taking place under the umbrella of a grouping called Unite to Remain, whose directors include Jim Knight, a former Labour MP who is now a peer, and Jessica Simor, a prominent remain-supporting QC.

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When the campaign was formally launched earlier in the month, the Lib Dems’ Heidi Allen, who announced on Tuesday she was standing down as an MP, said it hoped to cover 70 constituencies. This would involve the three parties standing down for each other and, in some places, not standing against strongly remain Labour candidates.

Sources said a formal announcement was expected in the coming days, probably next week. One party source said there had been a meeting several weeks ago, but none more recently. “I imagine a general election will concentrate people’s minds and the process will now move fairly quickly,” they said.

One complication is that decisions have to be agreed by local activists. The three parties involved are notably decentralised, with constituency parties having considerable autonomy. The source said: “The hope is that with such an uncertain election, and the tight timetable, we can get people to agree more willingly.”

The 2017 plan ran into problems, mainly because of a perceived unwillingness of parties to give ground, especially Labour. The Greens subsequently expressed scepticism about taking part in a similar process again.

But a Green spokeswoman said the party was committed to the process now: “It is right that political parties engage in grown-up discussions about how to best stop a deeply damaging Brexit. Our party is in talks about the potential to stand a single remain-supporting candidate in some seats in England and Wales, as was successfully done in the Brecon and Radnorshire byelection. No decisions have yet been made and any agreement will involve the relevant local parties.”