The SNP have just under 125,000 registered members compared to 124,000 for the Tories, according to House of Commons Library data.
“The latest available data shows that membership of the SNP (August 2018) has surpassed the latest reported figures for the Conservatives (March 2018),” a spokesperson for the House of Commons Library tweeted.
A Conservative Party source told The Independent: “We have been driving up membership steadily since the beginning of the year.”
Nicola Sturgeon reacted with amazement to official figures showing her party was now the second largest in the UK by membership.
“Wow – the SNP is now officially the second biggest party in the whole of the UK,” the Scottish first minister and leader of the SNP said.
SNP membership rose from 118,162 in April to 125,482 in August, according to information provided by the party’s headquarters.
However, the latest Conservative membership figure was reported in March and therefore does not account for any possible changes since then.
Labour had 540,000 registered members, the House of Commons Library data showed.
SNP business convener Derek Mackay MSP welcomed the figures, which he said were boosted in June by the row over the so-called Brexit “power grab” on devolved governments.
“Over 7,000 people joined the SNP in just five days in June, propelling us ahead of a waning Tory party which is at risk of imploding completely over Brexit,” he said.
“Like the extraordinary membership surge of 2014 joining the SNP has once again become not just a powerful symbol, but the best way to ensure Scotland’s voice is heard.
“People were rightly outraged at Tory plans to remove powers from the Scottish Parliament, and that only 15 minutes were given over at Westminster to debate the impact of the EU Withdrawal Bill on devolution.”
It comes as a poll revealed Brexit has damaged support for the British union in both Scotland and Northern Ireland.
Almost half of Scottish voters (47 per cent) said they would now vote for independence if the UK leaves the EU.
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