The Commerce Commission says many gyms may be breaking the law by letting customers pay for memberships by instalments.

Prompted by complaints about difficulty or expense of trying to cancel memberships, the commission investigated and found many gyms were likely to break the law by failing to inform customers of their right to cancel a contract and failing to disclose the full cost of the membership.

The customer had a right to full information when signing up for a credit contract, said the commission's competition branch manager Graham Gill.