If your employer provides you with access to sporting or recreational facilities (or vouchers that are exchangeable for their use), providing certain conditions are met the cost met by your employer is tax free.
The conditions are that the facilities are available for use by all of the firm's employees and the facilities aren’t available to the general public. In addition the facilities must be used mainly by employees or former employees, or by members of employees’ families and households.
The facilities don’t have to be used exclusively by your firm's employees – so if your employer has jointly offers the facility with another firm and their employees use the facilities too, the membership is still tax free.
The cost of the membership is not tax free where the facilities are open to the public, or based at a private residence or holiday accomodation, or where the membership provides you with use of a vehicle (which includes boats and aircraft).
In most cases employers secure preferential rates at public gym facilities, rather than offering to meet the full cost of the membership, as the provision of the membership will be a taxable benefit. If they pay your membership, you are then liable to pay tax on the full cost of providing you (and members of your family or household where appropriate) with the use of the facilities in question.