This article discusses a case where the question was answered if a person was acting as an agent or a principal in relation to the taxi business which he operates. The First Tier Tribunal found that when the person was negotiating the contracts with the local authorities there was no pre-determined driver, meaning that there was no relationship between the local authority and the driver. Therefore it was held that the person must be acting as a principal as there was no driver on whose behalf he was acting.
Source: constablevat.com
