The figures were revealed by LO-Tidningen, LO's in-house journal.
If the fall in union membership continues at the same pace, LO will have lost 100,000 members by the end of the year. The cost for LO will be 16-17 million kronor. The total cost to the confederation's 15 members is expected to top 300 million kronor.
The financial shortfall will lead to many LO officials losing their jobs.
Union leaders blame the government for the fall. Since coming to power last September, Prime Minister Fredrik Reinfeldt's centre-right coalition has abolished tax deductions on union membership fees. It has also forced up the fees in the union run, taxpayer funded unemployment insurance schemes.
A large number of members have quit the unions as a result of the increased costs, leaders say.
Pia Håkansson, treasurer of the Swedish Building Maintenance Workers Union (Fastighets), told LO-Tidningen that women and older people were losing out. She called the government's reforms "idiotic taxes."
LO's loss is unlikely to cause ministers to shed tears: the organization paid between 90 million and 100 million kronor towards the Social Democrats' election campaign, in addition to also actively campaigning on the party's behalf.