The Green Party has ruled out further "progressive alliances" with other left-wing parties after their efforts to band together were apparently rebuffed.
Co-leader Caroline Lucas said Labour and the Liberal Democrats had "betrayed the people they represent" by failing to properly consider electoral pacts.
She said Labour refused to even meet with the Greens to discuss possible alliances, while the Lib Dems were only willing to engage at arm's length.
Labour and the Lib Dems defied their own members on the ground and the time has now come for the Greens to focus entirely on winning votes up and down the country.
She also criticised Labour for expelling two senior local members for backing a GP standing for the National Health Action (NHA) party against Health Secretary Jeremy Hunt.
The longstanding South West Surrey activists Kate Townsend and Steve Williams were barred for supporting GP Louise Irvine in a seat where Labour is also fielding a candidate.
The Green Party will stand aside for Dr Irvine, with Labour and Lib Dem activists expected to offer her their support.
Ms Lucas said Labour's actions were "shortsighted" as the left had a chance to unite around a pro-NHS candidate but the party "might well have now blown it".
The Green Party has stood aside in a number of strategically important seats, boosting the chances of candidates such as Lib Dem Sarah Olney in Richmond and Labour's RupaHuq in Ealing.
While senior Labour and Lib Dem figures have supported these alliances at local level, both parties nationally have ruled out any deals.