The JJB Sports founder, Dave Whelan, is to acquire the majority of the 70 Fitness First UK sites in a £70m deal, and wants to combine them with his existing chain of 91 DW health clubs.
Two further buyers are involved in acquiring the small remainder of the branded gyms, The Gym Group and GLL, a charitable social enterprise established by Greenwich Council.
The rest of Fitness First's operations - in Asia, Australia and Germany - are likely to be sold separately once the UK deal is completed.
Once the UK's largest gym operator, Bournemouth-headquartered Fitness First has over 370 clubs and around one million members in 16 countries.
Leeds-based Pure Gym is now the largest operator in Britain following its acquisition of LA Fitness last year.