The Green Party says it now has more members than UKIP, after a 2,000-strong surge of new joiners in England and Wales overnight.
The party has a UK-wide total of 43,829 members compared with UKIP's 41,966.
The news is likely to renew the row over whether the Greens should be included in the TV election debates ahead of the 7 May general election.
David Cameron has said he will not take part in the leaders' head-to-heads unless the Greens are invited.
The proposed format for the pre-election live TV debates put forward by the broadcasters include the Conservatives, Labour, the Lib Dems and UKIP.
The BBC's Carole Walker said the membership figures would "add weight" to the Greens' argument that they should be invited to take part.
The SNP and Plaid Cymru, which have more seats in Parliament than either UKIP or the Green Party, also say they should be represented in any series of debates.
The Greens are made up of three parties, the largest being in England and Wales with 35,481 members.
Its sister parties in Scotland and Northern Ireland have 8,026 and 322 members, respectively. Students can join the Green Party for £5, with standard membership costing £31 a year.
The Conservatives say they now have 224,000 paying members - up 30% in a year - although it includes their £1-a-year "supporters". Last September the influential Conservativehome website put Conservative Party membership at 149,800. Under 23s can join for £5 a year, with standard membership costing £25 a year.
Labour says it had 189,531 members at the end of 2013. It costs £12 a year for those under 26, with standard membership costing £46.50 a year. Its figure does not include registered supporters, affiliated members or what it says "many thousands of small donors".
The SNP membership is now 93,000, after seeing a significant rise in membership - from 25,000 - following the Scottish referendum last year.
The Liberal Democrats had a membership of about 44,680 in their latest figures - last April - putting them just ahead of the Greens.
Broadcast regulator Ofcom suggested last week that UKIP should be treated as a major party but not the Greens.
The news was a blow to the party, as those which are granted the status are entitled to at least two party election broadcasts on each of the TV and radio channels covered by the system.
The Green Party of England and Wales came fourth in last year's European election, winning three seats and just under 8% of the vote.
It is fielding candidates in at least 75% of seats for the forthcoming general election.
Prime Minister and Conservative Party leader David Cameron has said he will not take part in the leaders' head-to-heads unless the Greens are invited.
Labour leader Ed Miliband has accused the PM of "running scared" while Lib Dem leader Nick Clegg and UKIP leader Nigel Farage say Mr Cameron is making excuses for not wanting to appear.
Natalie Bennett, Green Party in England and Wales leader, said any debate would be "unrepresentative" if the Greens were not included and that the party was talking to lawyers about possible legal action.
The BBC, Sky News, ITV and Channel 4 said they "remain committed" to staging debates before the poll on 7 May.