Vista - Credentials in an RDP file cause prompt to confirm trusted connect

Does anyone know what setting can be applied to the RDP file when you are
supplying credentials through an RDP file. When adding the credentials
through the lines

username:s:{USER}
domain:s:{DOMAIN}
password 51:b:{ENC_PASSWORD}

A prompt is shown to the user. We are trying to make the connection
transparent so would require to suppress any messages to the client.

Cheers