The Labour Party's membership is believed to have fallen below half a million people for the first time since its peak under Jeremy Corbyn.
The party had 201,293 registered members at the time of the 2015 general election, but experienced a huge influx after Ed Miliband stepped down and triggered a leadership race.
Hundreds of thousands of people were galvanised by Corbyn's anti-austerity, anti-war agenda and the membership rose to over 554,000 in the months following his election success, making Labour among the largest political parties in Europe.
But after repeated attempts to dethrone the left-winger and a failure to capture the imagination of the wider public in the polls, that tally has slowly eroded away with fears that some have lost faith in the leader.
A Labour source told the Guardian that the current membership number is estimated to be around 483,000 after 40,000 people fell behind on their monthly payments.
The figure was disclosed at Labour's National Executive Council on Tuesday (21 March), but it was not clear if the drop was caused by a change of heart or a simple failure to renew memberships.
However, the sudden decrease is unusual and is believed to be a direct result of poor performance. Another Labour source told the Guardian that many had left the party as a direct result of Corbyn's decision to whip MPs in favour of voting for Article 50.
A different theory suggests that the sharp drop was caused by Conservative voters, who joined Labour en masse to vote for Corbyn in order to "destroy" the party, and who have since failed to renew their membership. It follows a campaign by the Daily Telegraph urging readers to do so, taking advantage of Labour's £3 membership cost.
In spite of the decrease, Labour still remains overwhelmingly Britain's biggest party.
The Conservative Party had 149,000 as of 2013 data and the Liberal Democrats had roughly 82,000 members.
A Labour spokesman said the party does not comment on membership figures.