Probe launched into gym contracts
It is considering whether these companies are using or recommending unfair contract terms and engaging in unfair business practices in breach of regulations.
The watchdog is not naming the companies under investigation at this stage.
It said: "This investigation is at an early stage and it should not be assumed that the parties involved have breached any consumer protection legislation.
"The OFT will not reach a view on whether the law may have been infringed by any company until it has completed its investigation."