Shopworkers' union Usdaw now has 100,000 members signed up at market-leading grocer Tesco.
John Hannett, deputy general secretary and head of recruitment strategy at Usdaw, said: 'Nationally, Usdaw has increased its membership by 10,000 over the last 12 months and now has more than 318,000 members. We are the fastest-growing union in the UK and I'm confident we'll continue to grow in membership and influence.'
A Tesco spokesman said: 'Tesco has a unique relationship with Usdaw, which improves consultation and gives our people a voice within the company.'
Usdaw has signed 40 new agreements with companies in recent years, and believes there is potential to double in size in the near future.