Green Party membership appears to have surged past Ukip's amid the row over general election debates.
Some 2,000 more people joined overnight, bringing the total across the country to 43,829, according to a spokesman.
Ukip said its membership stood at 41,943 as of yesterday, while the latest figures issued by the Liberal Democrats in November put them only slightly higher on 44,526.
The Greens have been fighting to be included in the televised election debates alongside David Cameron, Ed Miliband, Nick Clegg and Nigel Farage.
The Prime Minister has made his own participation dependent on the party featuring, saying it would unfair to leave them out if Ukip are given a berth. Opinion polls have suggested the public also backs their inclusion.
But Ofcom has indicated that the Greens should not be treated as a major party for the purposes of campaign coverage, and the format currently proposed by the broadcasters does not involve leader Natalie Bennett.
Yesterday Ms Bennett wrote to Mr Miliband, Mr Clegg and Mr Farage urging them to accept her involvement so that the debates can go ahead.
The Greens' membership is spread across devolved branches in England and Wales, Scotland, and Northern Ireland.
They are still substantially smaller than the SNP - whose numbers have soared to around 92,000 in the wake of the Scottish independence referendum.
As of September, Tory membership was declared to be 149,800, while Labour's was around 190,000.
Plaid Cymru has around 8,000.
However, the figures should be treated with caution as the parties are not independently monitored.
Some have introduced different levels of membership with lower fees, while there have been reports of lapsed members being left on the books in order to maintain numbers.
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