Financial trade union Unifi is merging with Amicus, the UK's largest private sector union.
The new union will have a strong presence in the financial industry
The merger of Unifi's 140,000 members with Amicus will create a union with two-thirds of all trade union members in the UK finance industry.
Unifi, whose members voted nine-to-one in favour of the move, will be absorbed into Amicus, but its full-time employees will retain their posts.
Amicus has 1.2 million members, of whom 50,000 work in the finance industry.
'Work to do'
"We are all in effect being integrated into Amicus," a Unifi spokesman told BBC News Online.
"We have a big presence in the finance sector, and hope to strengthen that with the transfer into Amicus.
"There is a lot of work to do tacking issues in the financial industry."
The expanded union hopes to close gender pay gaps in the finance sector.
It will also campaign on pensions entitlements and preventing job losses on the back of outsourcing.
In the past six years there have been 33 mergers of UK trades unions.