The Liberal Democrats, Plaid Cymru and the Green Party have formed an electoral pact, agreeing not to stand against each other in dozens of seats.
The three anti-Brexit parties will announce details of the deal on Thursday, but it is thought to cover between 60 and 70 constituencies.
In Wales, the pact will cover 11 of its 40 seats, BBC Wales has been told.
Such a pact means two of the parties would agree not to field a candidate, boosting the third candidate’s chances.
Outside of Wales, the pact will simply be a two-way agreement between the Lib Dems and the Greens.
Thursday marks exactly five weeks until the UK general election on 12 December.
All three parties have already launched their election campaigns. They all support another Brexit referendum and