On Wednesday 30th July, Jeremy Corbyn issued an apology and a correction over his claims that half a million people had signed up to his new party’s mailing list. That was untrue, he said. The actual number was more than 600,000. That figure looks tantalising on paper and if 100% of those sign-ups converted into party memberships, Corbyn would be at the helm of the U.K’s largest political party. But how realistic is this prospect and what do the early numbers suggest is possible for the as-yet-unnamed left-wing group?
Sign-ups vs Memberships
So what does Jeremy Corbyn mean when he says that 600,000 people have ‘signed up’ to his new party? It is important to note that those who have expressed interest in the new party have simply done exactly that; expressed interest. A sign-up is not a paid membership – it is more akin to a mailing list, with donation being an optional choice for those who receive email updates. Rival political parties don’t generally measure size, support or momentum by signups and instead refer to paying members when discussing their outreach. So while this figure is an impressive indication of early momentum and curiosity, it does not necessarily mean Corbyn is set to challenge the Labour Party or even Reform UK on total membership.
What is Corbyn up against?
| Party | Paid‑up Membership (most recent data) |
| Labour Party | 309,000 (Feb 2025) |
| Reform UK | 227,600 (July 2025) |
| Conservative Party | 131,700 (Nov 2024) |
| Lib Dems | 90,000 (Early 2025) |
| Green Party (England & Wales) | 60,000 (March 2025) |
| (House of Commons Library, Canary) |
The Labour party currently boasts the highest number of paying members, but the recent rise of Reform UK has thrown the proverbial cat amongst the political pigeons. When Nigel Farage returned to the party as leader in June 2024, membership surged by 12.5% within a week, rising from 40,000 to 45,000. By the end of that year, Reform had overtaken the Conservative Party and now sits on 227,600 members – 81,400 behind Labour. If Reform continues to grow at this rate (an average growth of 11,086 members per month between December 2024 and July 2025), it will surpass Keir Starmer’s crew by March 2026. So what does Jeremy Corbyn need from his 600,000-strong army of intrigue if he is to do the same?
Conversion rate permutations: what does Corbyn need to match his rivals?
The simple fact is this: if Jeremy Corbyn’s new party is to become the UK’s largest, he needs just over half (51.5%) of his 600,000 sign-ups to convert into paid memberships. However, many commentators believe that the party’s formation will only serve to damage Labour by splitting the left vote and further sucking support from incumbent prime minister Keir Starmer. Net result? Socialist visionary Jeremy Corbyn potentially handing the 10 Downing Street keys over to bitter populist rival Nigel Farage. In order to quieten these conversations, Corbyn may first set his sights on reeling in Reform UK. If 38% of his sign-ups convert, he will do just that. A more conservative estimate (groan) would put conversion at around 15-20%, slotting the party in behind the Tories.
Can we make any predictions?
If every single one of the people who have already signed up to receive email updates from Corbyn and co decide to become fully fledged members, they would be almost twice the size of the Labour Party, dwarfing all rivals and cementing themselves as a major political force. However, these expectations may need to be tempered quite significantly. In 2015, Corbyn set up an activist vehicle named ‘Momentum’, a left wing organisation formed to support his bid for the Labour leadership. Much like his current venture alongside former Labour MP Zarah Sultana, Momentum aimed to transform the party into a more democratic and member-driven outfit. Here’s how the numbers shake out:
- The movement began with a ‘database of around 150,000 supporters’ in 2015.
- By April 2018, Momentum claimed 40,000 paying members – a conversion rate of 26.7% in 3 years.
- If we apply this to Corbyn’s 600,000, we’re looking at just over 160,000 paying members.
- This would see the party become the third largest in the UK by 2028, ahead of the Conservatives and behind only Labour and Reform.
Does membership guarantee votes?
All this talk about memberships is just talk unless the memberships that converted from sign-ups convert into votes (Alexa, play ‘Conversion’ by Nick Cave and the Bad Seeds). Research from LSE suggests that a minority of members still defect on polling day and that the number is higher for smaller/less established parties.
It is very possible that left-leaning voters will face a tough choice at the next election; vote for what they believe in or vote for what keeps Farage away from Downing Street. In tight battleground constituencies, the prospect of a tactical vote for Labour to deny Reform may cause some of Corbyn’s allies to defect – whilst a decision not to do so may hand the seat to Farage’s party. Plus, historical surges in party membership don’t always mean electoral success, and nobody knows this more than Corbyn himself. Under his leadership, Labour grew its membership to a staggering 550,000 but failed to win either of its elections. It’s also worth remembering that membership does not provide an accurate picture of the full electorate; according to research from the University of Oxford, party members tend to skew older, educated and male.
So while that 600,000 headline figure is an undeniable and extraordinary sign of early momentum, there is a long road ahead for Jeremy Corbyn, Zarah Sultana and the supporters of the UK’s latest political movement.