Contactless technology is widely used in security sensitive applications, including identification, payment and access-control systems. Near Field Communication (NFC) is a short-range contactless technology allowing mobile devices to act primarily as either a reader or a token. Relay attacks exploit the assumption that a contactless token within communication range is in close proximity, by placing a proxy-token in range of a contactless reader and relaying communication over a greater distance to a proxy-reader communicating with the authentic token. It has been theorised that NFC-enabled mobile phones could be used as a generic relay attack platform without any additional hardware, but this has not been successfully demonstrated in practice. We present the first generic practical implementation of a contactless relay attack by using only NFC-enabled mobile phones, requiring only suitable mobile software applications. This implementation reduces the complexity of relay attacks and therefore has potential security implications for current contactless systems.