The Liberal Democrats say they have brokered an electoral pact with Plaid Cymru and the Green Party not to field candidates against each other in at least 60 constituencies.
Details of the so-called Remain alliance will be announced at a press conference on Thursday by the Unite to Remain group, which helped to engineer the deal.
“We are delighted that an agreement has been reached,” said Lib Dem leader Jo Swinson. ”This is a significant moment for all people who want to support remain candidates across the country.”
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Ms Swinson has come under fire after she “absolutely categorically” ruled out working with Jeremy Corbyn, even if it meant delivering another referendum.
The agreement comes after the Lib Dems beat the Conservatives in the Brecon and Radnorshire by-election after the other two parties stood aside in a move partly engineered by Heidi Allen.
Following the Lib Dem’s by-election success, Ms Allen – a former Conservative, Change UK and Lib Dem MP – launched the non-partisan Unite to Remain.
“With a single remain candidate in 60 seats we will return a greater number of remain MPs to parliament,” Ms Allen told The Guardian.
“This is our opportunity to tip the balance of power away from the two largest parties and into a progressive remain alliance.”
In September, Tory chairman James Cleverly reported the pro-EU group to the Electoral Commission, claiming MPs planned to circumvent spending limits.
Mr Cleverly allegedly sent texts written by Ms Allen encouraging MPs to run as independent candidates via “shell parties” to the watchdog’s chairman Sir John Holmes.
In the wake of Mr Cleverly’s complaint, Ms Allen suggested at least 20 “One Nation” Tories could be willing to jump ship to the Liberal Democrats, however none did so.
An announcement on a pro-EU general election pact had been expected for some days. Recent polling has consistently suggested a majority of voters now favour remaining within the EU.
Additional reporting by PA