In Community Dental Centres Ltd v Sultan-Darmon, the EAT held that where an individual has an unlimited right to appoint a substitute to carry out services on his or her behalf (for any reason and without sanction) this will usually be fatal to the individual successfully claiming that they are a worker.

The claimant was a dentist who entered into a contract with Community Dental Centres Ltd to provide dental services. The company provided equipment and support staff to the claimant; introduced patients to the claimant; and required the claimant to work set hours. Such arrangements are common in the dental profession. The employment tribunal found that, as the claimant could decline to treat individuals referred to him by the company, there was insufficient mutuality of obligation to satisfy the test of employee. However, he did satisfy the lesser test of "worker". The EAT disagreed and noted that it was inconsistent that, having already found a lack of mutuality of obligation, the tribunal could then conclude that the individual was a worker. Furthermore, the claimant's contract with the company contained a clause allowing the claimant to nominate a substitute to carry out services on his behalf at any time when he was unable or unwilling to provide the services himself, and the claimant was not subject to any sanction from the company for exercising this right. The EAT concluded that this unrestricted right of substitution indicated that there was an insufficiently strong undertaking on behalf of the claimant to perform services personally. As such he did not satisfy the test of "worker".