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.”

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.

Top 10 marginal seats to watch at the general election Show all 10 1 /10 Top 10 marginal seats to watch at the general election Top 10 marginal seats to watch at the general election North East Fife Currently held by the SNP with a majority of 2 Getty Top 10 marginal seats to watch at the general election Kensington Currently held by Labour with a majority of 20 Getty Top 10 marginal seats to watch at the general election Perth and North Perthshire Currently held by the SNP with a majority of 21 Getty Top 10 marginal seats to watch at the general election Dudley North Currently held by independent, formerly Labour, MP Ian Austin with a majority of 22 LivingInMediocrity Top 10 marginal seats to watch at the general election Newcastle-under-Lyme Currently held by Labour with a majority of 30 Derek Harper Top 10 marginal seats to watch at the general election Southampton Itchen Currently held by the Conservatives with a majority of 31 Rob Candish Top 10 marginal seats to watch at the general election Richmond Park Currently held by the Conservatives with a majority of 45 Robin Webster Top 10 marginal seats to watch at the general election Crewe and Nantwich Currently held by Labour with a majority of 48 Jaggery Top 10 marginal seats to watch at the general election Glasgow South West Currently held by the SNP with a majority of 60 Alec MacKinnon Top 10 marginal seats to watch at the general election Glasgow East Currently held by the SNP with a majority of 75 Christine Johnstone

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.