The Liberal Democrats, the Greens and Plaid Cymru are in "negotiations" about electoral pacts in certain constituencies to ensure a pro-Remain candidate is elected.
Green Party co-leader Jonathan Bartley told the BBC that "negotiations are ongoing" with other parties, as campaigning for 12 December's election gets under way.
Mr Bartley said it was "no secret" that the Greens were "talking to the Lib Dems and Plaid" but "nothing has been finalised".
All three parties support another Brexit referendum and voting to Remain in the EU.
He claimed "a temporary electoral arrangement to stand aside for one another" would help elect "a big block of MPs that aren't from the two main parties".
Deputy Lib Dem leader Sir Ed Davey confirmed his party was taking part in pre-election discussions with Plaid Cymru and the Greens.
"They, recognising us as the strongest Remain party, want to see if there is things we can do with them," he told BBC Breakfast.
The MP for Kingston and Surbiton also repeated his party's promise not to enter into a coalition government deal with Labour or the Conservatives, saying Jeremy Corbyn and Boris Johnson were both "unfit to be prime minister".
He said the Lib Dems had "things in common" with Plaid and the Greens, "particularly on the Brexit issue" and they would work with these parties to defeat the Conservatives and Labour.
Plaid Cymru leader Adam Price said that the Welsh party was "still in discussion with the other pro-Remain parties and our local parties as well".
Mr Price said the parties hoped to find agreement on a deal in "the next few days".
Plaid Cymru and the Greens formed a pact with the Liberal Democrats at the Brecon and Radnorshire by-election in the summer, standing down their candidates in favour of the Lib Dems' Jane Dodds.
She went on to defeat the Conservative incumbent, Chris Davies.
The Conservatives would achieve a majority of 44 if no tactical voting takes place but anti-Brexit voters could stop this, according to a new study by the pro-EU Best for Britain campaign.
Conservative MP, and health secretary, Matt Hancock said: "A vote for Lib Dem is a vote to put Jeremy Corbyn in Downing Street."
Labour leader Jeremy Corbyn has ruled out forming any electoral pacts with other parties.
Speaking on a visit to a hospital in West Sussex, he said: "No electoral pacts, we're fighting this election to win it".
The Labour leader said that the party would be campaigning in every constituency on a manifesto that would "end austerity" and "transform our society".
Laura Parker, national co-ordinator of Momentum, the grassroots organisation made up of Labour supporters, warned Labour supporters about voting tactically on 12 December.
"We will be advising all of our members to get out there and campaign for Labour, wherever they are in the country.
"It's only a Labour government that will both end this Tory Brexit madness and end austerity and help us rebuild Britain."
Left-leaning candidates stood aside in 41 constituencies in the 2017 general election, however 38 of these candidates were from the Greens. Only two were Liberal Democrat candidates. No Labour candidates stood aside.
Electoral pacts have also become a topic of controversy in Northern Ireland as the Ulster Unionist Party (UUP) announced that they would stand in all 18 constituencies.
Unionist parties have traditionally agreed electoral pacts in certain constituencies in order to maximise the number of unionist MPs at Westminster.
The Democratic Unionist Party's (DUP) chief whip has called the move "bonkers".